Because of the booming development of offshore wind power around the world, a stable transmission system which is used for the connection between the offshore wind farms and the onshore grid is required. For the offshore wind farms not far from the coast, high voltage alternating current (HVAC) transmission system is the best choice. Aiming to study the transient problems caused by cable operation, a 60 km submarine cable is modeled in this paper using ATP-EMTP. The larger capacitance effect of HVAC submarine cables will cause more severe transient problems. Also, the variable wind power generated by offshore wind farm will bring undesired impact on the onshore power grid. This paper proposes a superconducting magnetic energy storage (SMES) system which can mitigate both the high frequency fluctuation of wind power and the transient over voltage of the HVAC cable system. In addition, SMES sizing study has been done to achieve the proposed functions.