Permanent magnet synchronous motor (PMSM) control can be divided into two categories: zero-low-speed and medium-high-speed control, in order to realize the full-speed domain control of permanent magnet synchronous motor, it is necessary to composite the two control methods, however, in the frequent wide-frequency domain speed regulation condition of the motor, the two control methods are frequently switched, which is easy to result in the inaccuracy of the estimated position and the rotational speed oscillation, and the two control methods are required to be separately designed and considered as algorithmic transition problems, which increases the system adjustment difficulty and complexity of the algorithm. The two control methods need to be designed and considered separately, which increases the system calibration difficulty and algorithm complexity. In this paper, a flux observer is designed according to the integral method of the back electromotive force (EMF), which can suppress the initial value error and DC bias in the inverse electromotive force, and can also be controlled in the full-speed domain, and simulation verification is carried out, which proves that the flux observer can be controlled in the full-speed domain.