The temperature monitoring of the stator winding is one of the important fundamental protections for permanent magnet synchronous motor (PMSM) used in electrical hydrostatic actuator of the aircraft and the joint of the robot. This paper proposed an improved method for stator winding temperature estimation based on sinusoidal alternating current injection. First, a simple device for injecting sinusoidal ac voltage into PMSM contributes to measuring stator winding resistance by the switching action of the device was developed. Compared to the existing monitoring methods, this method does not embed the extra component in monitored motor, so it is simple and effective in the practical realization. Then, the stator winding temperature is calculated through the strict linear relationship between metal resistance and its temperature. The analysis and experimental results has proved that the method is feasible for monitoring stator winding temperature of PMSM.