The main box double countershaft structure: the symmetrically arranged double countershafts balance the radial forces on the input shaft and the gears on the main shaft. The input shaft and the main shaft only bear the torque without bending deformation; the intermediate bearing is subject to bending moment reduction Small 1/2, small bending deformation, good gear meshing condition. Sub-box planetary mechanism: This mechanism has the characteristics of compact structure, light weight, large transmission ratio and high efficiency. Since one sun gear meshes with 5 planetary gears at the same time, the bearing capacity is greater.