Floating photovoltaic technology, as an emerging offshore solar solution, can assist coastal areas in transforming their energy structure, expand the utilization of marine resources, and has broad development prospects. It is expected to provide significant support for the country's “3060” strategy. In response to the current lack of research on the calculation methods for offshore floating photovoltaic power generation, this study proposes a calculation model for offshore floating photovoltaic power output based on wave motion and other marine environmental characteristics of the target marine area. Firstly, based on the theories of wave mechanics and the hydrodynamics of floating bodies, and considering the steepness of the target sea area Γ as a small value (wave height H / wave length L << 1) as a boundary condition, the study uses radiation energy efficiency η_A and the time-averaged tilt angle ε to quantify the long-term impact of waves on photovoltaic modules, resulting in an equivalent irradiance objective function for the inclined surface under regular wave action; Secondly, considering the effects of the back of the photovoltaic modules and cooling on the water surface caused by seawater reflection, evaporation, and convection respectively, a practical calculation formula for back irradiance and a cooling effect factor for the water surface are introduced to represent the positive gain from the cooling effects on the back of the photovoltaic modules and the water surface. Finally, a comparison with actual measurements at the target site shows that the calculated values differ from the theoretical values by less than 10%, which is within the expected variance limit. Therefore, this study has significant reference value for the development and improvement of offshore floating photovoltaic technology, providing ideas for the estimation of expected returns in subsequent related marine engineering projects.