The highperformance control of permanent magnet synchronous motors requires accurate information of the speed and rotor position. This information can usually be measured by mechanical sensors such as resolvers and photoelectric encoders. However, it is difficult to use highperformance mechanical sensors since their high price, difficulty in installation, and poor adaptability to harsh environments. By measuring the electrical signal of the motor, the position of the rotor and the motor speed information can be calculated based on the obtained electrical signal and the motor’s parameters. In this paper, based on the relationship between motor back electromotive force and motor speed, a sliding mode observer based on back electromotive force was designed to estimate the motor rotor position and motor speed, so as to realize the speed sensorless control of surfacemount permanent magnet synchronous motor.