By adopting the high frequency pulse injection method, a research has been conducted on the sensorless control of permanent magnet synchronous motors. The method of quadratic harmonics is used to identify the rotor polarity in order to improve the convergence of the algorithm. Based on Matlab/Simulink, a simulation test has been carried out of the high frequency injection method of PMSM, double loop speed and current closed loop control. The results show that the rotor position of PMSM with small saliency ratio is difficult to demodulate, with a great jitter in the torque. Therefore, when the pulse frequency injection method is used, the PMSM with higher saliency ratio will be able to achieve a better control result.