When asymmetric faults occur in the AC transmission lines of offshore wind networks, the reliability of traditional differential protection is reduced because of the effect of capacitive current in the negative sequence network, as the converters on both sides of the line adopt negative sequence suppression control strategies. To address this issue, this paper first outlines the structure of the offshore wind power transmission system and analyzes the fault characteristics through the control strategies of power electronic equipment on both sides of the transmission line. Considering capacitive current, the adaptability of longitudinal differential protection and the blind area of natural frequency differential protection are then evaluated. On this basis, using the negative sequence voltage of the offshore MMC busbar and the sum of the positive sequence currents on both sides of the line, a location-indicating variable  is constructed and is further used to construct an auxiliary protection criterion. Finally, an offshore wind power system model is built in PSCAD/EMTDC to analyze and verify the effectiveness of the proposed protection scheme. The results show that the proposed method exhibits high reliability, fast response, and can tolerate 20 dB noise and 100  fault resistance.