In order to solve the over current problem of MMC-HVDC during AC fault, while enhance the low voltage ride through capability of the MMC converter, this paper finds that the converter active and reactive power control mode is decoupling PI control through researching converter power mathematical model and control mode. And it proposes a kind of emergency power support control strategy, which controls the PI value to limit the power output, at the same time, generates positive and negative sequence current compensation through the deviation of the AC voltage RMS when symmetric or asymmetric faults occur in AC side. The control strategy is added to the electromagnetic transient simulation system. When symmetric and asymmetric faults occur in network side, power quality of the valve side is analyzed by using numerical simulation method. Simulation results verify the validity of fault over current suppression of the proposed control method, at the same time it can enhance the low voltage converter through ability. This work is supported by Science and Technology Development Project of State Grid Corporation of China (No. SGRIZLKJ [2015]457).