The invention discloses a method for determining the DC (direct current) running range of a high-voltage DC transmission system in an AC (alternating current) fault. The probability of subsequent commutation failures of a high-voltage DC can be reduced, AC fault recovery performance of the high-voltage DC transmission system can be effectively improved, and the method has great significance for improvement of stability of an AC and DC hybrid transmission system. The method comprises the following steps: solving upper limit values of DC command values in three-phase symmetrical and asymmetricalfaults in real time according to a voltage-current characteristic function, calculating reactive power compensated by an AC filter based on an instantaneous power theory in real time, determining anexpected value of the reactive power consumption of a converter by setting reactive power exchange capacity between the AC system and a converter station to a certain expected value, solving the current command value under the working condition, and finally, taking the smaller value and determining the final command value of the DC through a limiting link. Therefore, the probability of subsequentcommutation failure of the high voltage DC can be reduced, AC fault recovery performance of the high voltage DC transmission system is effectively improved, and the method has great significance for improvement of stability of the AC/DC hybrid transmission system.