For the magnetic saturation and cross coupling effect, the dq-axis inductance parameters of the permanent magnet assisted synchronous reluctance motor (PMASynRM) are unstable. In order to adapt the instability of the inductance and improve the robustness of the system, an improved algorithm of sliding mode observer (SMO) position sensorless control is proposed, which is based on the extended back electromotive force. The proposed control can estimate the position of the motor. To improve the chattering resistance ability of the system, the super twisting (ST) SMO is used to replace the conventional SMO, and the sigmoid function is used to replace the conventional sign function. The phase-locked loop (PLL) in rest frame is used to obtain the rotor position information. An improved SMO sensorless vector control simulation model is built in MATLAB/Simulink. The simulation analysis verifies that the improved algorithm can estimate the rotor position quickly and accurately, which has good chattering resistance and dynamic and static quality.