Abstract: At the communication moment of brushless DC motor（BLDCM）,the terminal voltage is distorted by diode freewheeling current from which phase current will be turn off.Taking the back-EMF zero-cross detection method in sensorless position control,this terminal voltage distortion can make position detection signal phase ahead,deviating from the best commutation time,especially,in heavy-load condition it can cause commutation failure.Therefore back-EMF zero-cross detection method has been restricted in light-power application.The mechanism of freewheeling current causes advance signal has been analyzed in this paper,and based mathematical model,a position detection signal phase ahead compensation method is presented too.Simulation and experimental results show that the compensated signal is close to the best commutation time,BLDC motor can still be normal operation by sensorless control with heavy-load.