For Permanent Magnet Synchronous Motors(PMSM) in a time-varying disturbance environment, the sliding mode control with better robustness is adopted, an improved reaching law is proposed based on the traditional exponential reaching law in sliding mode control. Based on the improved reaching law, a sliding mode speed controller of PMSM speed regulation system is designed to improve the running performance of the motor. Through comparative analysis of simulation results, the sliding-mode speed controller based on the improved reaching law can not only improve the dynamic performance of the system, but also weaken the chattering phenomenon of the system, and it can make the motor still have high running performance under time-varying disturbance.