The modular multilevel converter (MMC) is one of the most promising converter topologies for medium/high-power applications. The upper and lower arm parameters of MMC have some differences inevitably in the practical projects. Moreover, the differences of resistance will become big due to different losses and number of faulted sub-modules among different arms. In [10], the impact of MMC under asymmetric parameter conditions was briefly introduced and it pointed out that the dc current and double-frequency current will flow into the ac side, the fundamental-frequency circulating current will flow into the dc side. In [11], a control strategy with asymmetric arm parameter was proposed, which could eliminate the circulating current and keep the arm capacitor voltage balance. However, the dc current might appear in the ac side of the converter. To solve these problems, the decoupled control model of MMC under asymmetric parameter conditions was established. On this basis, the arm current direct control strategy was then proposed in order to improve the performance of current control of MMC. Meanwhile, an optimized control method of arm capacitor voltage balance was proposed, which can keep the terminal behavior of MMC unaffected by the circulating current.