In the sensorless control system of permanent magnet synchronous motors(PMSMs), the high robustness of the sliding mode observer has been a research hotspot. To solve the problems of large amounts of calculation of the traditional sliding mode observer (SMO) and its high-frequency chattering, a new SMO for the medium and high speed sensorless control process of PMSM is designed. The boundary layer and phase-locked loop (PLL) technology are applied in SMO, and the mathematical model of SMO is built. The design of PLL is strictly deduced, and the precision of rotor position is reasonably verified. Finally, the design is implemented by simulation and experiment. The results show that the proposed new SMO has good suppression of high-frequency chattering. Simultaneously, the precision and robustness of the new SMO are higher.