Abstract: For the oscillating float type wave energy capture device, the maximum capture power, resonance frequency and capture bandwidth are usually used as indicators to evaluate the buoyancy capability of the float. However, these three indicators are difficult to achieve optimal performance at the same time, and because they do not contain sea state information, they cannot reflect the applicability of the float under different sea conditions. In this paper, combining with speed and the optimal damping theory, the definition of derived a new float can ability evaluation index, the index contains float water dynamic response and the spectral information of ocean waves, can float energy capture performance at the same time and the energy distribution of sea condition, can more fully reflect certain waters float to performance. In order to verify the practicability of the evaluation index, this paper takes the new evaluation index 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 index 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.