The multi speed gear reducer with small tooth difference is a new type of transmission device which integrates the common cylindrical gear transmission, planetary gear transmission and small tooth difference transmission technology. The reducer adopts double shaft input (equipped with double motors) and composite output mode. In the case of using non adjustable speed motor, it can realize multiple speed output of the reducer. In the case of using single or two speed regulating motors, it can realize stepless speed regulation of the reducer, with a wide range of speed regulation. The reducer also has the characteristics of large transmission ratio, large bearing capacity, strong overload resistance, high transmission efficiency, large load ratio, compact structure, high safety factor and high cost performance ratio.