Description Waterproof Joint Technical Field of the Invention The present invention relates to a waterproof joint. Background of the Invention In daily life, when in use of a low-voltage electrical apparatus, connection joints need to be used due to a far distance from a power supply and the lack of a long enough wire of the electrical apparatus. However, a majority of the existing joints are not waterproof, so it is likely to result in short circuit and damage the circuit boards, caused by the entrance of water, and it will also form threat to personal safety. Such problems are much more likely to occur particularly when the existing joints are used outdoor. Summary of the Invention In view of the problems in the prior art, an object of the present invention is to provide a technical solution of a waterproof joint. A waterproof joint is provided, including a male contact and a female contact, wherein the male contact consists of a male contact connector, a nut, an O-shaped seal ring, a male contact plug jacket and a male contact plug, one end of the male contact connector being connected to a wire while the other end thereof being provided with the male contact plug jacket, the outer diameter of the male contact plug jacket being less than that of the make contact connector, the O-shaped seal ring being fixedly disposed at a connection part of the male contact plug jacket and the male contact connector; the nut is sheathed outside the male contact connector, one end of the nut being of an wide-mouth structure while the other end thereof being of a necked 1 structure, the wide-mouth end of the nut being a connection end, the inner diameter of the wide-mouth end of the nut being greater than the outer diameter of the O-shaped seal ring, the necked end of the nut being a limiting end, the inner diameter of the necked end of the nut being less than the outer diameter of the O-shaped seal ring; the male contact plug jacket is provided therein with the male contact plug, a gap being provided between the male contact plug and the male contact plug jacket, the inner surface of the male contact plug jacket being combined by a chambered surface and planar surface; the female contact consists of a female contact connector, a female plug jacket and a female plug cavity, one end of the female contact connector being connected to a wire, external threads fitted with the connection end of the nut being provided on the outer surface of the other end of the female contact connector, the female contact plug jacket being provided inside the female contact connector with the external threads provided on one end, the female contact plug jacket being provided therein with the female plug cavity fitted with the male contact plug, the structure of the female contact plug jacket being fitted with that of the inner surface of the male contact plug jacket; and, the male contact plug jacket is plugged between the female contact connector and the female contact plug jacket, the male contact plug being plugged in the female plug cavity, the female contact connector being in contact fit with the O-shaped seal ring, the nut being in threaded fit with the external threads on the female contact connector. Preferably, an anti-slip strip is provided on the outer surface of the nut. In the present invention, the circuit may be connected through the plug-in of the male contact and the female contact. The O-shaped seal ring disposed at the connection part of the male contact and the female contact may play a function of sealing. Moreover, the threaded fit of the male contact nut with the external threads on the female contact connectors further ensures the reliability of connection and the tightness of the joint, so that it is ensured that 2 no water will enter the joint. Brief Description of the Drawing Fig. 1 is a structure diagram of a male contact; and Fig. 2 is a structure diagram of a female contact. Detailed Descrition of the Invention The present invention will be further described as below with reference to the accompanying drawings. A waterproof joint is provided, including a male contact and a female contact. The male contact consists of a male contact connector 1, a nut 2, an O-shaped seal ring 3, a male contact plug jacket 4 and a male contact plug 5. One end of the male contact connector 1 is connected to a wire, while the other end thereof is provided with the male contact plug jacket 4. The outer diameter of the male contact plug jacket 4 is less than that of the make contact connector, so that it ensured that the male contact plug jacket can be plugged in the female contact. The O-shaped seal ring 3 is fixedly disposed at a connection part of the male contact plug jacket 4 and the male contact connector 1. When the male contact is connected to the female contact, the connection is sealed by the O-shaped seal ring 3. The nut 2 is sheathed outside the male contact connector 1. One end of the nut is of a wide-mouth structure while the other end thereof is of a necked structure. The wide-mouth end of the nut is a connection end, and the inner diameter of the wide-mouth end of the nut is greater than the outer diameter of the O-shaped seal ring; and, the necked end of the nut is a limiting end, and the inner diameter of the necked end of the nut is less than the outer diameter of the O-shaped seal ring, so that it is ensured that the nut can be sheathed on the O-shaped seal ring and in screwed connection with the female contact. The male contact plug jacket 4 is provided therein with the male contact plug 5. A gap 6 is provided 3 between the male contact plug 5 and the male contact plug jacket 4 and used for plugging a female contact plug sleeve 8. The inner surface of the male contact plug jacket 4 is combined by a chambered surface and planar surface. The non-arc inner surface may function as limiting the male contact plug jacket 4 and the female contact plug sleeve 8. The female contact consists of a female contact connector 7, a female plug jacket 8 and a female plug cavity 9. One end of the female contact connector 7 is connected to a wire. External threads 10 fitted with the connection end of the nut are provided on the outer surface of the other end of the female contact connector 7. The female contact plug jacket 8 is provided inside the female contact connector 7 with the external threads provided on one end. The female contact plug jacket 8 is provided therein with the female plug cavity 9 fitted with the male contact plug 5. The structure of the female contact plug jacket 8 is fitted with that of the inner surface of the male contact plug jacket 4. The male contact plug jacket 4 is plugged between the female contact connector 7 and the female contact plug jacket 8. The male contact plug 5 is plugged in the female plug cavity 9. The female contact connector 7 is in contact fit with the O-shaped seal ring 3. The nut 2 is in threaded fit with the external threads 10 on the female contact connector. An anti-slip strip may also be provided on the outer surface of the nut 2, so that no slip is ensured during the threaded connection. 4