In order to improve the dynamic quality of the permanent magnet synchronous motor (PMSM) speed control system, a new approach law is proposed. Based on the traditional exponential approach rate, a compound function with all system state variables is introduced into the constant velocity term to suppress the buffeting of sliding mode control. The sliding mode approach speed is improved. Secondly, in order to avoid the phenomenon of integral saturation, it is combined with the inverse computational anti-saturation method. Based on this approach law, the sliding mode speed regulator of a permanent magnet synchronous motor is designed. The simulation results are compared with the PI regulator and conventional exponential approach law respectively. The results show that the speed controller can effectively improve the system's static and dynamic characteristics and robustness.