Brushless DC motor (BLDCM) drive system is applied in multi-electric/all-electric aircrafts to provide power output for electric fuel pump, cooling pump and electric brake. In order to meet the special working environments such as complex electromagnetic interference and extreme temperature, a sliding mode back electromotive force (EMF) observer based on variable structure control strategy is proposed to realize the position sensorless control of motor. The variable reaching law is used to replace the fixed gain of the traditional sliding mode back EMF observer, and the rotor position and speed information of the motor are obtained through the phase-locked loop (PLL). The simulation results show that the method can effectively suppress the chattering of the observed value of back EMF, realize the stable operation of the motor in the middle and high speed stage, and meet the requirements of rapidity and robustness of the system.