To improve the observation accuracy of sensorless control of a permanent magnet synchronous motor (PMSM), a sensorless control method based on backstep robust control and an improved tracking differentiator is proposed. First, the mathematical model of a PMSM is established. Second, based on the Sigmoid function, a tracking differentiator is designed to realize the observation of the speed and position of the PMSM rotor. Then, to improve the anti-interference ability of the system, the backstep and robust controls are combined, and the backstep robust controller is designed for the PMSM speed regulation system. Finally, a hyperbolic tangent tracking differentiator is designed to observe the external load in real time and to compensate the control system to reduce the influence of load disturbance on the motor speed. The results show that the observer has a good observation effect on the speed and position of the motor rotor, and the backstep robust control also has high control accuracy and a good inhibitory effect on the load.