Aiming at the problems of high chattering, slow convergence speed and poor observation accuracy of sliding mode observer in permanent magnet synchronous motor position sensorless control system, a new sliding mode observer based on adaptive sliding mode gain variable power reaching law is proposed. Sliding mode observer, this reaching law adds an adaptive sliding mode gain variable exponential term consisting of the observed rotational speed, flux linkage and external input expected target current error on the basis of the power reaching law. The exponential term has a fast time-varying convergence rate, which effectively solves the problem that the original reaching law takes too long to reach the modal. The stability of the system is analyzed by Lyapunov stability criterion. Control experiments were carried out with the new sliding mode observer and the traditional sliding mode observer, the results show that the proposed new sliding mode observer has faster convergence speed, it can weaken highfrequency chattering and enhance load carrying capacity, and the adaptive sliding mode gain can effectively control the current error amplitude, Finally, the observation accuracy is improved by more than 20 percent.