The invention discloses a method and a system for judging converter station AC (alternating current) outlet faults of a flexible DC (direct current) transmission system. The method is implemented in line protection and comprises steps as follows: parameters for the line protection are newly added, the protection of the side can be optionally set as protection of a converter side or a side oppositeto a converter; for the line protection of the converter side, when negative-sequence current is lower than a set threshold value, zero-sequence current is higher than the set threshold value and thezero-sequence current and the negative-sequence current meet a mutual proportional relation, the line protection of the side is judged as a forward fault; for the line protection of the side oppositeto the converter, when the negative-sequence current is higher than a set threshold value and the zero-sequence current is lower than the set threshold value, the line protection of the side is judged as the forward fault; the line protection of the two sides is subjected to information interaction, and when the forward faults are determined on both sides, the fault of the line area is determined. The method can be suitable for the converter station AC outlet application scenes of the flexible DC transmission system, and protective action performance of the line is improved.