In order to reduce the cogging torque of interior permanent magnet synchronous motor (PMSM) for electric vehicles, a method of stator tooth shoulder chamfering was proposed in this paper, the equivalent model of air gap length before and after the chamfer of stator tooth shoulder was established, and the effective air gap length distribution function was deduced, so as to analyze the mechanism of stator tooth shoulder chamfering reducing the low harmonic amplitude of air gap magnetic density and weakening the cogging torque. To explore the optimal shapes and sizes of the chamfer based on a PMSM with three phase 36-stator-slot/8-rotor-pole topology, the finite element method was purposely utilized for conducting optimization. The comparative analysis results show that the elliptic chamfer of stator tooth shoulder can effectively reduce the amplitude of air gap magnetic dense harmonic wave, improve the sinusoidal waveform of motor back EMF and weaken the cogging torque. The peak value of the cogging torque of the optimized motor is reduced by 77.2%, the amplitude of the 9th, 13th, 15th, 17th, 19th and 21st harmonics of the back EMF is significantly reduced, and the output quality of the motor is significantly improved.S