An improved sliding mode observer (SMO) was designed to realize sensorless control in wide speed range of permanent magnet synchronous motor (PMSM). A back electromotive force (EMF) observer was used to replace the low pass filter, which avoided the problem of the rotor position information lag. Meanwhile, the feedback value of back EMF were introduced into the calculation of stator current and feedback coefficient according to the variation of rotating speed of the rotor was adaptively adjusted, which reduced the chattering of the system. Thus the rotor position and speed estimation was more accurate in wide speed range. The phase locked loop (PLL) technique was used to suppress the high frequency information to extract the accurate rotor position. Based on the dSPACE rapid control prototype experimental platform, the sensorless control system of static and dynamic experiments were carried out. The results verified the validity of the proposed method.