1. ENCLOSING WALLBOARD CONNECTION STRUCTURE Field of The Technology The invention relates to an enclosing wallboard connection structure, and belongs to the field of courtyard protection. Background of the invention Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field. Along with continuous improvement of social economy, people have higher requirements on courtyard protection. However, construction and design of most of existing courtyard enclosure or fences are relatively original and uncreative. Enclosing wallboard manufacturing and mounting processes are complex and extremely low in mounting efficiency. Moreover, site construction is time-consuming. Existing wooden enclosing wallboards on the market usually adopt a mortise and tenon structure which is constructed at the site. It has disadvantages of being inefficiency, high labour consumption and unreasonable construction cost. Although factory production is adopted for manufacturing the traditional plastic-wood enclosing wallboards sold on the market, the mounting and fixing still has many shortcomings, such as insecure connection after being constructed at the site, damage to the original structure during secondary use, resource waste and negative impacts on environmental protection. Summary of the invention It is the objective of the preferred embodiments of this invention to overcome or ameliorate one or more of the shortcomings of the prior art and to provide a novel enclosing wall board connection structure capable of being assemble without a skilled worker. According to the invention there is provided an enclosing wallboard connection structure having an enclosing wall, a column body and a door, wherein the column body is mounted on the enclosing wall and the enclosing wall consists of a plurality of enclosing wallboards. A lower end cap is mounted at the lower end of the enclosing wall, a connecting piece is mounted on the lower end cap, and the enclosing wallboards are inserted into the connecting piece. A projection extends beyond the upper end of each enclosing wallboard, groove designed to receive the projection is formed at the lower end of corresponding enclosing wallboards, so the enclosing wallboards are combined together by connecting the projection and groove. A 2. fence is mounted at the upper end of the enclosing wall, protection strips are respectively mounted on the upper and lower edges of the fence. A connecting piece is mounted at the upper end of the protection strip which is on the upper edge of the fence, an upper end cap is mounted at the upper end of the connecting piece, and the connecting piece is inserted into the upper end cap. The protection strip on the lower edge of the fence is mounted on the enclosing wallboards through the connecting piece. A plurality of oblique grooves are formed in the protection strip, the protection strip is mounted on the fence through the oblique grooves. U-shaped grooves are formed on four lateral surfaces of the column body, and the enclosing wallboards are inserted into the U-shaped grooves. Preferably, a projection extends beyond one side of each enclosing wallboard, a groove is formed in the other side of each enclosing wallboard, and the enclosing wallboards are mounted together in a left-right direction by inserting the projection into the groove. Preferably, the door is connected with the column body through hinges. Preferably, the door consists of a doorframe and a door sheet, the doorframe is fixed on through L-shaped connecting pieces, and the door sheet is directly fixed on the doorframe. Preferably, the connecting piece comprises a component a and a component b, the component a is shaped with two tooth-like projections and the upper and lower transverse branches of the component a are respectively provided with a tooth-like projection, grooves configured to accommodate the tooth-like projections are formed in the component b, and a rectangular groove is formed at the lower end of the component b. Preferably, the upper end cap comprises a master piece and two sub pieces, each sub piece is shaped with two tooth-like projections and the upper and lower transverse branches of each sub piece are respectively provided with a tooth-like projection, grooves configured to accommodate the tooth-like projections are formed in the left and right sides of the master piece, and a rectangular groove is formed at the lower end of the master piece. Preferably, a vertical protection strip is arranged on the right side edge of the fence, a vertical connecting piece is arranged outside the vertical protection strip, the vertical connecting piece is connected with the connecting piece at the upper end of the fence through an L-shaped connecting piece, one end of the L-shaped connecting piece is inserted into the connecting piece, and the other end of the L-shaped connecting piece is fixed on the vertical connecting piece through a bolt. Preferably, a base is arranged at the lower end of the column body, the base is positioned underground, the base consists of four embedded parts, the base has a square cross section, the embedded parts are fixed together through bolts, a lower sealing buckle is mounted at the lower end of the base, and the lower sealing buckle is square to be matched with the cross section of the base. Preferably, an embedded part end cap is arranged at the upper end of the base, a buckle is 3. arranged on one side of the embedded part end cap, and the embedded part end cap is positioned on the ground. Preferably, the L-shaped connecting piece is made of aluminium or rigid plastic, and the door and the column body are made of aluminium or rigid plastic. The enclosing wallboard connection structure disclosed in the preferred embodiments of this invention has advantages of being time-saving, labour-saving, convenient to mount and transport and firm in mounting. The bolts are used so that the enclosing wallboard connection structure can be reused. Hence the structure has broad market prospect and huge potential economic benefits. Compared with the prior art, the cost of the connection structure is also being reduced with an environment-friendly manner, and aesthetics and practicability are combined as a whole. The enclosing wallboard connection structure can be directly assembled at the site because all parts are durable, convenient to transport, and simple to mount, thereby being time saving and more practical. Unless the context clearly requires otherwise, throughout the description and the claims, the words "include", "including", "comprise", "comprising" and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to". Brief description of the drawings The preferred embodiments of the invention will now be described in more detail, and by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of the first embodiment of the enclosing wallboard connection structure. Fig. 2 is a schematic diagram of the fence. Fig. 3 is a schematic diagram of the column body. Fig. 4 is a schematic diagram of the second embodiment of the enclosing wallboard connection structure. Detailed description of the embodiments Embodiment 1 As shown in Fig. 1, Fig. 2 and Fig. 3, the enclosing wallboard connection structure includes the enclosing wall 1, the column body 2 and the door 3, wherein the column body 2 is mounted on the enclosing wall 1 and the door 3 is mounted on the column body 2. The enclosing wall 1 is made of wood or wood-plastic material, and the door 3 and the column body 2 are made of aluminium or rigid plastic material. All of them can be dismounted and conveniently 4. transported. The enclosing wall 1 consists of a plurality of enclosing wallboards 11 to facilitate assembly and transportation. A lower end cap 4 is mounted at the lower end of the enclosing wall 1, a connecting piece 5 is mounted on the lower end cap 4, and corners of the lower end cap 4 are connected with the enclosing wallboard 11 through L-shaped connecting pieces to mount firmly. A projection is arranged on the connecting piece 5, a groove is formed in the lower end cap 4, and the connecting piece 5 is inserted into the lower end cap 4. The projections 111 extend beyond the upper ends of the enclosing wallboards 11, grooves 112 designed to receive the projections 111 are formed at the lower ends of the enclosing wallboards 11, the enclosing wallboards 11 are combined together through the connection of the projection 111 and the grooves 112. A fence 6 is mounted at the upper end of the enclosing wall 1, protection strips 61 are mounted on the upper and lower edges of the fence 6, the protection strip 61 on the lower edge of the fence 6 is mounted on the enclosing wallboards 11 through the connecting piece 5, a plurality of oblique grooves 611 are formed in the protection strip 61, and the protection strip 61 is mounted on the fence 6 through the oblique grooves 611. The connecting piece 5 is mounted at the upper end of the protection strip 61 on the upper edge of the fence 6, an upper end cap 7 is mounted at the upper end of the connecting piece 5, and the connecting piece 5 is inserted into the groove of the upper end cap 7, so that stability in connection is ensured. The door 3 is connected with the column body 2 through hinges to move flexibly. The door 3 consists of a doorframe 31 and a door sheet 32, the doorframe 31 is fixed through L-shaped connecting pieces, and the door sheet 32 is directly fixed on the door frame 31. The connecting piece 5 includes a component a and a component b, the component a is shaped with two tooth-like projections like a Chinese character "Gong", upper and lower transverse branches of the component a are provided with tooth-like projection , grooves matched with the tooth-like projection are formed in the component b, and a rectangular groove is formed at the lower end of the component b, into which the enclosing wallboards 11 or the protection strips 61 can be conveniently inserted. The upper end cap 7 includes a master piece 71 and two sub pieces 72, each sub piece 72 is shaped with two tooth-like projections like a Chinese character "Gong", upper and lower transverse branches of each sub piece 72 are provided with tooth-like projection, grooves matched with the tooth-like projection are formed respectively on the left and right sides of the master piece 71, and a rectangular groove is formed at the lower end of the master piece 71, into which the connecting piece 5 can be conveniently inserted. In order to ensure firm connection, a vertical protection strip 62 is arranged on the right side edge of the fence 6, a vertical connecting piece 8 is arranged outside the vertical protection strip 62 to connect with each other through groove. The vertical connecting piece 8 is connected with the connecting piece 5 at the upper end of the fence 6 through an L-shaped connecting piece, one end of the L-shaped connecting 5. piece is inserted into the connecting piece 5, and the other end of the L-shaped connecting piece is fixed on the vertical connecting piece 8 through a bolt. Ingenious design and firmness in connection are achieved. In order to ensure mounting firmness of the column body, a base 21 is arranged at the lower end of the column body 2, the base 21 is positioned underground, the base 21 consists of four embedded parts c, the base 21 has a square cross section, the embedded parts c are fixed with each other together through bolts, a lower sealing buckle 211 is mounted at the lower end of the base 21, and the lower sealing buckle 211 is square and matched with the cross section of the base 21. An embedded part end cap 22 is arranged at the upper end of the base 21, a buckle is arranged on one side of the embedded part end cap 22, and the embedded part end cap 22 is positioned on the ground. The structures are precisely designed to ensure firmness of the column body 2 and facilitate mounting. The L-shaped connecting piece ismade of aluminium or rigid plastic material, and the door and the column body are made of aluminium or rigid plastic material. U-shaped grooves are respectively formed on four lateral surfaces of the column body 1, the enclosing wallboards are inserted into any one of the U-shaped grooves, and the other grooves may be sealed. A ruler can be selectively mounted at the lower end of the column body 2 with an exposed scale part to facilitate depth measurement for a mounting worker with such simple design. In addition, a wiring channel can be selectively formed on the lower edge of the fence 6or in the column body 2.Wires penetrate through the wiring channels, and electricity transformers may be arranged on the enclosing wallboards 11 through the wiring channels, so that various lamps, holiday colour lamps, infrared sensors, monitors, intelligent alarming systems and the like can be arranged on the enclosing wall 1. The transformers may be shaped as birdcages or other forms. Embodiment 2 As shown in Fig. 2, Fig. 3 and Fig. 4, an enclosing wallboard connection structure includes the enclosing wall 1, the column body 2 and the door 3, wherein the column body 2 is mounted on the enclosing wall 1, the door 3 is mounted on the column body 2, the enclosing wall 1 is made of wood or wood-plastic material, and the door 3 and the column body 2 are made of aluminium or rigid plastic material. All of them can be dismounted and conveniently transported. The enclosing wall 1 consists of a plurality of enclosing wallboards 12to facilitate assembly and transportation. A lower end cap 4 is mounted at the lower end of the enclosing wall 1, a connecting piece 5 is mounted on the lower end cap 4, a projection is arranged on the connecting piece 5, a groove is formed on the lower end cap 4, and the connecting piece 5 is inserted into the lower end cap 4. The projection 121 extends beyond the upper ends of the enclosing wallboards 12, grooves 122 are formed at the lower ends of the enclosing wallboards 12, and the enclosing wallboards 12 are mounted together in a left-right direction through the connection of the projection 121 and the grooves 122. A fence 6 is mounted at the upper end of the enclosing wall 1, protection strips 6. 61 are mounted on the upper and lower edges of the fence 6, the protection strip 61 on the lower edge of the fence 6 is mounted on the enclosing wallboards 12 through the connecting piece 5, a plurality of oblique grooves 611 are formed in the protection strip 61, and the protection strip 61 is mounted on the fence 6 through the oblique grooves 611. The connecting piece 5 is mounted at the upper end of the protection strip 61 on the upper edge of the fence 6, an upper end cap 7 is mounted at the upper end of the connecting piece 5, and the connecting piece 5 is inserted into the groove of the upper end cap 7, so that stability in connection is ensured. The door 3 is connected with the column body 2 through hinges to move flexibly. The door 3 consists of a doorframe 31 and a door sheet 32, the doorframe 31 is fixed through L-shaped connecting pieces, and the door sheet 32 is directly fixed on the door frame 31. The connecting piece 5 includes a component a and a component b, the component a is shaped with two tooth-like projections like a Chinese character "Gong", upper and lower transverse branches of the component a are provided with tooth-like projection, grooves matched with tooth-like projection are formed in the component b, and a rectangular groove is formed at the lower end of the component b, into which the enclosing wallboards 12 or the protection strips 61 can be conveniently inserted. The upper end cap 7 includes a master piece 71 and two sub pieces 72, each sub piece 72 is shaped with two tooth-like projections like a Chinese character "Gong", upper and lower transverse branches of each sub piece 72 are provided with tooth-like projection, grooves matched with the tooth-like projection are formed respectively on the left and right sides of the master piece 71, and a rectangular groove is formed at the lower end of the master piece 71, into which the connecting piece 5 can be conveniently inserted. In order to ensure firmness in connection, a vertical protection strip 62 is arranged on the right side edge of the fence 6, a vertical connecting piece 8 is arranged outside the vertical protection strip 62 to connect with each other through groove. The vertical connecting piece 8 is connected with the connecting piece 5 at the upper end of the fence 6 through an L-shaped connecting piece, one end of the L-shaped connecting piece is inserted into the connecting piece 5, and the other end of the L-shaped connecting piece is fixed on the vertical connecting piece 8 through a bolt. Ingenious design and firmness in connection are achieved. In order to ensure mounting firmness of the column body, a base 21 is arranged at the lower end of the column body 2, the base 21 is positioned underground, the base 21 consists of four embedded parts c, the base 21 has a square cross section, the embedded parts c are fixed with each other together through bolts, a lower sealing buckle 211 is mounted at the lower end of the base 21, and the lower sealing buckle 211 is square and matched with the section of the base 21. An embedded part end cap 22 is arranged at the upper end of the base 21, a buckle is arranged on one side of the embedded part end cap 22, and the embedded part end cap 22 is positioned on the ground. All the structures are precisely designed to ensure firmness of the column body 2and 7. facilitate mounting. The L-shaped connecting piece is made of aluminium or rigid plastic material, and the door and the column body are made of aluminium or rigid plastic material. U shaped grooves are respectively formed on four lateral surfaces of the column body 1, the enclosing wallboards are inserted into any one of the U-shaped grooves, and the other grooves may be sealed. A ruler can be selectively mounted in the column body 2 with an exposed scale part to facilitate depth measurement for a mounting worker with such simple design. The enclosing wallboard connection structure disclosed in the preferred embodiments of this invention has advantages of being time-saving, labour-saving, convenient to mount and transport and firm in mounting. The bolts are used so that the enclosing wallboard connection structure can be reused. Hence the structure has broad market prospect and huge potential economic benefits. Compared with the prior art, the cost of the connection structure is being reduced with an environment-friendly manner, and aesthetics and practicability are combined as a whole. The enclosing wallboard connection structure is directly assembled at the site because all parts are durable and convenient to transport, and also easy to mount, thereby being time saving and more practical. The embodiments only describe the technical concept and characteristics of the invention with an aim to enable those skilled in the art to understand the contents of the invention and implement the invention on such a basis but it is not intended to limit the scope of protection of the invention. Any equivalent variation and modification made according to the essence of the contents of the invention shall fall within the scope of protection of the invention.