It is difficult to estimate the low-speed starting position of permanent magnet synchronous motor (PMSM) when there is no speed sensor in the full speed range. Aiming at the problem, a control method combining constant current frequency conversion (I/F) and extended Kalman filter (EKF) is presented. At low speed, the constant current variable frequency ratio is adopted for starting. At medium and high speed, the extended Kalman filter is switched to estimate the position and speed. In order to solve the problem of unstable switching process, the self-balancing characteristics of motor torque and power angle is used. In the switching process, the angle difference given by the two control methods is used as the feedback input signal to reduce the given current before switching. When the angle difference is close to zero, the given current and position are switched, so that the given current and given position remain basically unchanged before and after switching. Finally, the feasibility of this method is verified by simulation.