In this study, the structure of the trailing edge of the vertical axis offshore wind turbine blade is modified. First, according to the method of parameterization, the offshore wind turbine model is established, and a series of characteristics of the offshore wind turbine are obtained. Second, we add flaps with different lengths to the trailing edge of NACA0021 airfoil to obtain different dynamic characteristics. The angle of the additional trailing edge flaps is modified. Finally, a simulation model for the modified airfoil of the vertical axis offshore wind turbine is reestablished, and the variable characteristics of the performance is studied. Through the optimization and analysis of the blade structure, this study has obtained the best parameters of the length and angle of the offshore wind turbine blade trailing edge flap. The optimization of the blade structure changes the flow field around the blade, which significantly improves the maximum wind energy capture rate and self-starting ability of the vertical axis offshore wind turbine.