In view of the drawbacks of application of AC power transmission technology and traditional HVDC transmission technology in the offshore wind farms and to meet the needs of ensuring the normal power supply on the island, a strategy that the flexible multi-HVDC transmission system based on MMC which can be used in the wind farm is adopted. The mathematical model of VSC-HVDC is established under the d-q rotating coordinate system, and the control methods for converter stations are designed. Simulation results show that the VSC-MTDC system is good at the connection between offshore wind farms and grid. When the power generated by wind farm is insufficient, the grids supply power to the load on the island. This way of grid connecting is flexible, and it is the best way for the connection between the wind farm on the island and land-based power grids.