The invention discloses an electronic phase change detection circuit of a position sensor-free brushless direct current motor, which is characterized in that a back electromotive force zero crossing point detection circuit detects a three-phase back electromotive force zero crossing point and sends the detect results isolated by a photoelectric isolating circuit into a microprocessor unit for further processing; a motor circuit, a detection circuit and the microprocessor unit are isolated from one another by the photoelectric isolating circuit, so that the interference generated by electric signals in the motor circuit and the detection circuit to an input signal of a microprocessor is prevented, the work stability and the system reliability of the microprocessor unit are enhanced, and a back electromotive force phase position is compensated through phase compensation information presorted in the microprocessor unit; and the back electromotive force zero crossing point and the phase change processing are determined, and further, the position of a rotor is determined without the adoption of the position sensor, so that the effect of the position sensor on the system reliability in the special environments of high temperature, high voltage, high interference, high vibration and the like is avoided, and further, and the system reliability is ensured.