A control method for controlling a synchronous motor includes calculating d-axis and q-axis current commands according to a frequency command and frequency of the synchronous motor by a MTPA control unit; when a feedback output voltage of the synchronous motor is larger than a control level, outputting a flux-weakening current command by a voltage control unit; calculating a flux-weakening current feed-forward value according to the frequency, a target level and the q-axis current command by a feed-forward control unit; when the sum of the flux-weakening current command and the flux-weakening current feed-forward value is smaller than the d-axis current command, adjusting the d-axis current command by the sum of the flux-weakening current command and the flux-weakening current feed-forward value; and outputting d-axis and q-axis voltage commands according to the adjusted d-axis current command and the q-axis current command to control the synchronous motor.