The position sensorless control system of permanent magnet synchronous motor is the research hotspot in the field of motor control. For the commutation control of motor without Hall sensor,sliding mode control was used to to detect the zerocrossing point of back electromotive force and complete the commutation of the motor. In order to obtain accurate back electromotive force observation values and attenuate the chattering phenomenon of sliding mode control,a new switching function was constructed instead of the traditional symbol function. The gain in sliding mode control was calculated using Lyapunov stability theory. In order to solve the rotational speed calculation,the adaptive algorithm was used to calculate the rotational speed because the derivative differential link affected the PMSM rotational speed control. Through simulation and experimental analysis,it is verified that the proposed scheme can accurately obtain the PMSM back electromotive force and rotational speed,and realize PMSM precise commutation control.