This paper studies the fault characteristic of MMC-HVDC connected with offshore wind farm based on the offshore AC system. Firstly, To analyze the influence mechanism of MMC control strategy to the systems voltage, then analyzed the characteristics of short-circuit current at the fault point and the mechanism of overvoltage generation, considering the impact of wind farm phase-locked loop, MMC circulation controller, and positive and negative sequence separation on the control system, on this basis, a direct voltage control strategy for MMC on the sending side is proposed, by modifying the d-axis and q-axis current control commands and combining voltage reduction coordinated control from two perspectives to resolve the impact of DC power surplus, overvoltage, and overcurrent on the wing field in flexible and direct systems; Finally, build a flexible DC transmission model for offshore wind power in PSCAD, which can be verified to effectively eliminate overvoltage and accelerate the voltage recovery speed after cutting off faults, implementing suppression strategy effects and maintaining safe system operation.