In order to reduce the cogging torque of IPMSM (Interior Permanent Magnet Synchronous Motor) symmetrical auxiliary slots are opened on the rotor surface. Firstly, the cogging torque is analyzed theoretically to find out the factors affecting the cogging torque of this motor. Conclusions are drawn from the finite element simulation analysis of the shape, position angle and radius of the auxiliary slot. Finally, the semi-circular auxiliary slot with position angle θ=38deg and radius r=0.3mm is used, and finally the waveforms of the cogging torque and no-load reverse potential of the motor are compared before and after cogging. The cogging torque can be weakened by 56.93% through the use of appropriate auxiliary slot parameters, as indicated by the simulation results, and the validity of the scheme is also confirmed.