Based on the OC3-Hywind Spar platform of NREL 5MW offshore wind turbine, the finite element model of the platform was established. The structural strength of the Spar platform under extreme sea conditions was analyzed by finite element method. Under the condition of satisfying the structural strength requirement, the optimization design method of the response surface is used to optimize the internal structure of the platform with the weight of the platform structure as the objective function. The optimization results show that the structural strength of the optimized platform is in accordance with the design specification. The structural dimensions of various ribs and stiffeners inside the platform are more reasonable and the amount of steel used in the platform structure is reduced, which reduces the construction cost of the Spar platform.