In order to weaken the jitter vibration of the traditional observer in the positionless sensor control of the permanent magnet synchronous motor, the segmented function is first designed to replace the switching function in the traditional sliding mode observer, and the stability of the Lyapunov stability criterion is used to prove its stability, and finally the new sliding mode observer is compared with the control system of the traditional sliding mode observer, and the results show that the new sliding mode observer has obvious effect on suppressing jitter vibration, and also improves the anti-interference ability.