Combined with the definition of meaningful velocity and optimal damping theory, a new evaluation indicator of float energy capture performance is derived, which contains the water dynamic response of float and the spectral information of ocean waves, and can simultaneously characterize the float energy capture performance and the energy distribution of sea condition, can more comprehensively reflect the energy capture performance of the float in specific sea area. In order to verify the practicability of the evaluation indicator, the new evaluation indicator is taken as the objective function to carry out the optimization design of float structure. The results show that the optimal design of the float structure with the new indicator as the objective function can improve the buoyancy capability of the float in the specific sea area and simplify the optimization process of the float parameters.