Based on the mechanism of concentric permanent magnet gear, a new model of low speed and high torque permanent magnet compound motor was designed, and the calculation method of the basic parameters of the model was given. By controlling the variable method, the parameters such as the length of the pole block, the thickness of the stator punch and the thickness of the inner rotor yoke were changed, and the parameters were optimized by Ansoft finite element simulation software. The results showed that the motor torque density and reduce speed fluctuations were effectively improved.