Aiming at the problems of high frequency chattering of sliding mode and large estimation error of rotor position in sensorless control system of permanent magnet synchronous motor (PMSM), an improved adaptive sliding mode observer (ASMO) is proposed based on the traditional sliding mode observer (SMO). The stability of the observer is proved by Lyapunov theorem. By using the piecewise exponential function instead of the traditional switch function and combining it with phase-locked loop (PLL) technology to extract the rotor position and speed signals from the back electromotive force (EMF), the chattering can be effectively suppressed and the observation error can be reduced. The simulation results of MATLAB show that the improved observer has higher accuracy and better dynamic performance than the traditional SMO.