Position sensorless control was the preferred choice for air conditioning compressor control systems. Aiming at the problems of poor zero speed observation accuracy and high starting difficulty of the position sensorless control system of permanent magnet synchronous motor (PMSM) based on sliding mode observer (SMO), an improved PMSM position sensorless control strategy was proposed, which could switch from the zero speed and low speed starting to SMO control under medium and high speed. At the low speed and zero speed stage, I/F control was used to keep the motor work in the angle openloop and current closedloop condition. When the angle error was too large to reach the system switching requirement, the phase difference of the real angle was reduced by reducing the openloop current. The angle of the SMO was compensated when the motor speed was increased, so that the observation angle was as close as possible to the real angle. In this way, the switching between the two algorithms became more stable, and the antiinterference ability was stronger. Through the MATLAB/Simulink simulation and the actual 3.5 kW vehiclemounted air conditioner motor debugging, the feasibility and effectiveness of the method were verified.