The invention relates to an alternating-current commutation failure detection-based alternating-current line fault direction discrimination method. According to the method, whether commutation failureoccurs on an inverter is detected; the working state of a directional element is judged; a fault direction is determined; if a direct-current system undergoes commutation failure during a time periodcorresponding to the directional element, protection is locked; and the problem of fault spread caused by alternating-current direction protection refusal or misoperation due to commutation failure can be solved.