For sensorless field oriented control of permanent magnet synchronous motor, a method of fluctuating high frequency square wave injection is used. Novel square wave voltage signal is injected by this method. High frequency response is extracted through a second-order finite impulse response (FIR) high pass filter. Rotor position is estimated by a closed-loop phase locking loop. The initial rotor polarity is distinguished by changing the current of d axis. Practical features of the control are expressed by formulas through theoretical analysis. Experiments are done according to the features. When the square wave is injected, the frequency of the wave can increase to one half of the control frequency and audible noise will basically disappear. Meanwhile, signal processing is enormously simplified, and the measurable range of rotation speed is improved to medium and high speed.