To solve the problem of the base speed change of the permanent magnet synchronous motor (PMSM) caused by the DC bus voltage drop during operation, a fluxweakening control method by adjusting the daxis current of the PMSM based on the DC bus voltage was proposed. When the bus voltage of the motor was lower than the bus voltage required for a given speed as well as the voltage limit ellipse intersected the current limit circle, the daxis current of the motor was adjusted by judging the voltage vector from the current regulator output value and the realtime DC bus voltage, so as to maintain the constant speed. The simulation model was built on MATLAB/Simulink according to the proposed control method. The simulation results verified that the proposed control method could solve the problem of sudden speed change caused by the DC bus voltage drop. Finally, an experimental platform was constructed to validate the effectiveness of the fluxweakening control method.