Abstract: A control strategy for modular multilevel converter (MMC) with energy storage ability is proposed for such flexible DC transmission application as power system connected with offshore wind farm. In the main circuit, battery banks are set in each sub-model of MMC. The DC terminal and AC terminal of converter are equivalent to controlled voltage sources, and feed-in power at DC side is smoothed in the region of energy storage capacity. During operation, the decoupled control of active power current at AC side of converter can realize AC power control, and the direct current control at DC side of converter can regulate DC power. In addition, SOC control methods of sub-model and sub-model cluster are given to balance the battery SOC in system. The simulation results show that proposed system can realize power control and storage function of AC/DC current system, and its viability is also verified.