This kind of gear box adopts two-grade parallel axis structure, which reduces the linear velocity of the gear pitch circle and improves the stability of the transmission. The input axis uses the sliding bearing that is relatively insensitive to the error and reduces the processing requirement. The center gear basically eliminates the axial force on the gear parameter configuration, which therefore adopts lower-cost ball bearing. The output axis adopts higher-reliability cone bearing so as to balance the axial force of output axis.