A sensorless vector control strategy of interior permanent magnet synchronous motor (IPMSM) based on sliding mode variable structure control (SMVSC) is studied. Aiming at the chattering problem of SMVSC, the generation mechanism is analyzed and studied in detail, then the saturation function is used to replace the sign function to decrease the discontinuity of sign function. At the same time, the border layer thickness parameter of the saturation function is set to be a self adaptive variable, and the value of the border layer thickness is adjusted to slow the speed of state variables approaching sliding mode switching surface to decrease the chattering problem. The simulation results prove that the proposed control strategy is effective. It has excellent dynamic and static performances, the estimated position and speed have good precision，and the system has strong robustness.