In recent years, the construction of long-distance and large-capacity offshore wind farms are developing very fast. The VSC-HVDC based on the modular multilevel converter (MMC) is the preferred option for the future long-distance offshore wind power interconnection. It will improve reliability and transmission power of offshore wind farm interconnection via bipolar MMC-HVDC system. Firstly, the bipolar MMC-HVDC system for offshore wind farm interconnection is introduced in this paper, and the improved control strategy of the bipolar MMC-HVDC system is designed detaily to realize the bipolar power balance and non-error control of the voltage and frequency of AC system. Finally, the offline simulation model is built on PSCAD/EMTDC platform to verify the effectiveness and feasibility of the above control strategies.