The invention provides a method and system for determining direct current interruption time after the alternating-current fault of the sending end of a hybrid direct-current transmission system. According to the method and system, a valve-side line voltage which is generated after the alternating-current fault of the sending end of the hybrid direct-current transmission system is determined according to the alternating-current voltage drop degree of the sending end of the hybrid direct-current transmission system and pre-input parameters; and the direct current interruption time after the alternating-current fault of the sending end of the hybrid direct-current power transmission system is determined according to the valve-side line voltage after the alternating-current fault of the sending end of the hybrid direct-current transmission system and the pre-input parameters. The calculation of the direct current interruption time is realized based on electric quantities obtained before and after the alternating-current fault of the sending end of the hybrid direct-current transmission system. With the method adopted, no additional devices are needed; the direct current interruption time after the alternating-current fault of the sending end of the hybrid direct-current transmission system can be effectively calculated; and realization cost is low.