A modular multilevel converter (MMC) has the problem of unbalanced power in asymmetric fault conditions. The conventional control method based on DC circulating current injection will cause asymmetry of arm current, resulting in unequal current of each phase. The method based on zero-sequence voltage injection may lead to over modulation and endanger the safe and stable operation of the system. Given the limitations of traditional leg-balancing control strategies, an adaptive leg-balancing control strategy of an MMC with asymmetric faults is proposed. First, the principle of leg-balancing based on zero-sequence voltage and DC circulating current injection is analyzed, and the limitations of different methods are pointed out. Second, the overmodulation boundary of the zero-sequence voltage injection method is studied, and the phase power distribution coefficient is introduced. Then the optimization method for a leg-power distribution coefficient with different faults is given and an adaptive leg-balancing control strategy of the MMC based on the coordination of zero-sequence voltage and DC circulating current is proposed. Finally, the correctness and effectiveness of the mechanical analysis and the proposed control strategy are verified by simulation.