The internal characteristics and system operation of MMC on the fault side are greatly affected when an asymmetric fault occurs in the AC power grid of MMC-HVDC based on a modular multilevel converter. Based on the average value model of the MMC bridge arm, this paper proposes an optimal control strategy for the MMC-HVDC system under asymmetric conditions, which enhances the fault ride-through capability of a flexible HVDC system. The strategy is mainly composed of two parts: AC current measurement control and circulating current suppression.On the AC side, the phase-locked loop based on a double second order generalized integrator (DSOGI-PLL) is used to extract the positive and negative sequence components of voltage and current accurately under asymmetric conditions, and the negative sequence current is suppressed with a double vector controller to control the three-phase current bal? ance on the AC side. The Proportional Integral and Repetitive Control（PI-RC）composed of PI controller and repetitive controller in series is used in MMC to suppress the positive and negative zero sequence components of double frequency in circulating current, so as to realize the constant transmission of DC power. Finally, a simulation model of the MMCHVDC system is built in MATLAB/Simulink software to verify the efficiency of the proposed optimal control strategy.