Description Motor vehicle rear axle differential mechanism Technical Field The invention relates to a component part of a motor vehicle, in particular to a rear axle differential of the motor vehicle, which is driven by double rear wheels or driven by four wheels. Background of the Invention The description of an invention patent which was authorized and announced by China's State Intellectual Property Office on June 24, 2009 and in the publication number of CN201261391Y disclosed a rear axle differential assembly of a four-wheeled motorcycle, a left half axle and a right half axle were respectively mounted in a left rear axle housing, a right rear axle housing, a left differential seat and a right differential seat through bearings, the left differential seat and the right differential seat were respectively sheathed in the left rear axle housing and the right rear axle housing through differential seat bearings, two planet gears were arranged between the left differential seat and the right differential seat through the bearing, the left and the right differential seats were connected through a bearing gland bush, the two planet gears were simultaneously meshed with two half axle gears, the two half axle gears were connected at the axle ends of the left and the right half axle, the structure took a differential as a planet carrier, the rotational inertia was great, and the larger driving force was required; in addition, the planet gears were arranged between the left and the right differential seats, the fluctuation of the planet gears was increased and the stability and other working performances of the differential were also affected. Summary of the Invention The problem to be solved by the invention is to provide a rear axle differential of a motor vehicle, which is simple in structure and convenient to mount, and can drive the differential to work through smaller driving force, reduce the mechanical energy consumption, improve the utilization efficiency of the driving force of an engine, further improve the stability of the differential and the smoothness and the operability of the motor vehicle, effectively avoid the occurrence of slippage and even side-turning phenomenon of the motor vehicle during turning, and improve the safety performance of the motor vehicle. In order to solve the technical problem, the invention adopts the following technical scheme: the rear axle differential of the motor vehicle is characterized by comprising a housing, as well as an input shaft gear component, a differential structure, a left output shaft and a right output shaft, which are arranged in the housing, wherein the differential structure comprises a planet carrier, two half axle gears and at least two planet gears, the planet carrier is a large gear sheathed at the right shaft end of the left 1 output shaft, the large shear and the left output shaft can rotate relatively, the large gear is meshed with the input shaft gear component for transmission, the planet gears are arranged in the large gear in a rotating manner and symmetric relative to the axial line of the large gear, and the planet gears are respectively meshed with the half axle gears. Further, the housing comprises a left housing and a right housing, the left housing and the right housing are fixedly connected through a screw, the left output shaft is arranged in the left housing by penetration, the right output shaft is arranged in the right housing by penetration, and the left output shaft and the right output shaft are arranged on the same axial line. Further, the left output shaft is longer than the right output shaft. Further, the input shaft gear component is a gear shaft meshed with the large gear, the left end of the gear shaft is fixed on the left housing through a bearing and connected with a driving device, and the right end of the gear shaft is fixed on the right housing through the bearing. Further, the half axle gears comprise a bevel gear I arranged on the left output shaft and a bevel gear II arranged on the right output shaft, the planet gears comprise a bevel gear III and a bevel gear IV, the bevel gear III and the bevel gear IV are positioned in a wheel hub of the large gear through a rotating shaft for horizontal rotation, and the bevel gear III and the bevel gear IV are respectively meshed with the bevel gear I and the bevel gear II for transmission. Further, the rotating shaft is a pin shaft, a shaft sleeve is arranged on the pin shaft, and the bevel gear III and the bevel gear IV are arrange on the shaft sleeve by penetration. The pin shaft is small in size and convenient to mount. After the technical scheme is adopted, the invention has the following advantages: the housing is fixed, and the large gear is taken as the planet carrier and sheathed on the left output shaft, thereby simplifying the planet structure, being convenient to mount, further improving the stability of the differential and the smoothness and the operability of the motor vehicle, effectively avoiding the occurrence of slippage and even side-turning phenomenon of the motor vehicle during turning, and improving the safety performance of the motor vehicle. Brief Description of the Drawings In combination of the following figures, the invention is further described. Figure 1 is a schematic diagram of structure of one embodiment of the invention. Detailed Description of the Invention According to a schematic diagram of the structure of one embodiment of the invention as shown in Figure 1, a rear axle differential of a motor vehicle comprises a housing 1 fixed on a rear axle, and an input shaft gear component 2, a differential structure, a left output shaft 31 and a right output shaft 32 are arranged in the housing 1; the housing 1 comprises a left housing 11 and a right housing 12, the left housing 11 and the right housing 12 are fixedly connected through a screw 13, the left output shaft 31 is arranged in the left housing 11 by penetration, the right output shaft 32 is 2 arranged in the right housing 12 by penetration, the left output shaft 31 and the right output shaft 32 are arranged on the same axial line, and the left output shaft 31 is longer than the right output shaft 32; the differential structure comprises a planet carrier 4, a bevel gear I 41 arranged on the left output shaft 31 and a bevel gear II 42 arranged on the right output shaft 32, as well as a bevel gear III 43 and a bevel gear IV 44, which are arranged on the planet carrier 4; the planet carrier 4 is a large gear sheathed at the right shaft end 311 of the left output shaft 31, the large shear and the left output shaft 31 can rotate relatively, the input shaft gear component 2 is a gear shaft, the large gear is meshed with the gear shaft for transmission, the left end 21 of the gear shaft is fixed on the left housing 11 through a bearing and connected with a driving device, and the right end 22 of the gear shaft is fixed on the right housing 12 through the bearing; and the bevel gear III 43 and the bevel gear IV 44 are positioned in a wheel hub 40 of the large gear for horizontal rotation and symmetric relative to the axial line of the large gear, and the bevel gear III 43 and the bevel gear IV 44 are respectively meshed with the bevel gear I 41 and the bevel gear II 42. In the embodiment, the number of the planet gears is two, and the number of the planet gears can also be four, six or eight; in addition, the rotating shaft 5 can adopt a pin shaft, a shaft sleeve 51 is arranged on the pin shaft, and the bevel gear III 43 and the bevel gear IV 44 are arrange on the shaft sleeve 51 by penetration. During the linear driving of the motor vehicle, driving wheels on the left side and the right side have almost the same resistance, power outputted from an engine is firstly transferred to the gear shaft, then the power is transferred from the gear shaft to the large gear to enable the large gear to start to rotate, the planet gears, namely the bevel gear III 43 and the bevel gear IV 44 in the large gear can perform revolution along with the large gear without the generation of rotation, the two planet gears, namely the bevel gear III 43 and the bevel gear IV 44 are meshed with the two half axle gears, namely the bevel gear I 41 and the bevel gear II 42 for realizing the rotation at the same speed, and then the motor vehicle can realize the linear driving; if the motor vehicle turns left, the driving distance of the driving wheel on the left side is short, the greater resistance is relatively generated, the large gear is connected with an input shaft, and the rotational speed is constant under the situation that the rotational speed of a transmission shaft is constant, so that the half axle gear, namely the bevel gear I 41 on the left side rotates slower than the large gear, the situation is equivalent to that the bevel bear III 43 needs to drive the left output shaft in a more labor-consuming manner, then the planet gears, namely the bevel gear III 43 and the bevel gear IV 44 can generate the rotation, and the more torque is transferred onto the half axle gear, namely the bevel gear II 42 on the right side; as the bevel gear III 43 and the bevel gear IV 44 perform the rotation in addition to the revolution, the bevel gear II 42 can increase the speed on the basis of the rotational speed of the large gear, then the right wheel rotates faster than the left wheel, and the vehicle realizes the smooth left turning; and the right-turning principle of the motor vehicle is the same with the left-turning principle. The housing is fixed, and the large gear is taken as the planet carrier and sheathed on the left output shaft, thereby simplifying the planet structure, being convenient to 3 mount, driving the differential to work through smaller driving force, reducing the mechanical energy consumption, improving the utilization efficiency of the driving force of an engine, further improving the stability of the differential and the smoothness and the operability of the motor vehicle, effectively avoiding the occurrence of slippage and even side-turning phenomenon of the motor vehicle during turning, and improving the safety performance of the motor vehicle. In addition to the preferred embodiment, the invention further has other implementation ways, those skilled in the art can make various changes and transformations according to the invention, and the changes and the transformations shall belong to the range defined by the attached claims of the invention on the premise of not separating from the spirit of the invention. 4