Aiming at the problem of sensorless control in the full speed domain of permanent magnet synchronous motor, it is difficult to estimate the position of the observer when the motor starts. A sensorless control strategy based on current-frequency ratio combined with Romberg observer is proposed. First, in the low-speed domain, I/F control is used to start; in the medium-high speed domain, Romberg observer and phase lock loop are introduced for position estimation. Secondly, to solve the oscillation problem caused by direct transition between two control methods, a new smooth transition strategy is proposed. For the problem of poor coordination and anti-interference ability of PI regulator, ADRC is introduced and combined with a new smooth transition strategy to form a compound transition method. Finally, through Simulink and motor experiment platform verification, simulation and experiment results show that the transition process of the composite control method is stable and the speed overdrive and error are significantly reduced.