In order to further suppress the instability of permanent magnet synchronous motor (PMSM) speed regulation system, a new exponential reaching law is designed based on the traditional law. A new function with sliding surface s as variable is added to the constant velocity term of the traditional exponential reaching law. At the same time, the traditional symbolic function is optimized and the curve is smoothed. The existence and the accessibility of the improved reaching law are proved. Then the improved law is applied to constructing the sliding mode speed controller and building a systematic Simulink simulation model. The simulation result is compared with that of PI control and the traditional exponential reaching law method. It is shown that the improved reaching law can reduce chattering and improve anti-interference performance, with more remarkable effect than the other two methods in all aspects.