It is easy to cause current shock and speed fluctuation, even switching failures for the permanent magnet brushless fan adopting traditional switching method such as angle-average switching, when using current-frequency (IF) open-loop starting method to switch to the position sensorless closed-loop algorithm using flux observer. An IF start smooth switching method based on cosine function in the switching process was proposed for the above problem. The optimal torque current is maintained in the switching process using the average filtering method, and the flux angle switches according to the law of cosine function by motor's torque power angle self-balancing characteristic, which achieves the effect of smooth actual current transition and improves the motor speed stability in switching process. The experimental results of the angle average switching and the proposed smooth switching method were compared to verify the effectiveness of the switching method. Results show that the method has strong adaptability to unstable loads and can improve the smoothness and success rate of the IF startup switching process.