This paper presents a sensorless Field-Oriented Control (FOC) method based on an improved sliding mode observer (SMO) for the sensorless control of permanent magnet synchronous motor (PMSM). The study begins by introducing the PMSM structure and its mathematical model, followed by an explanation of the FOC. Through the Simulink platform, the simulation model of PMSM, the FOC control algorithm, and the observer model based on the sliding mode observer and Hall sensor are respectively built. By improving the internal filter and sliding mode gain of the sliding film observer, adaptive observation of rotor speed and angle is realized. The observation effects of traditional Hall sensor observer, standard sliding mode observer and improved sliding mode observer at different speeds are compared through simulation. The results show that the improved sliding mode observer has better observation effects in medium-high speed operation, low speed operation and speed change. Especially at medium speed, it can effectively improve the estimation accuracy of electrical angle and speed, and significantly suppress the chattering phenomenon, and has better anti-interference ability and robustness.