In this paper the three-dimensional numerical simulation is conducted for Oscillating Wave Surge Converters (OWSCs) using computational fluid dynamics (CFD) software FLOW-3D, based on the wave climate of the offshore sea in northeast Taiwan, that is, 1.5 meters of wave height and 7 seconds of the wave period. The results are compared with that using the wave energy converter simulation toolbox, WEC-Sim. The effects of the parameters, including the flap thickness, flap width, the position of the mass center, and flap density, are investigated, especially their influence of the energy capturing efficiency or the capture factor (CF). The simulation results show that, bigger flap thickness (d), smaller flap width (B), smaller flap density, and lower flap’s center of mass will result in a higher efficiency for power capture performance. Among the four parameters, the flap thickness is the most dominant parameter. From hydrodynamic respect, a larger drag force may occur at the sidewalls of the thick flap, and the shear force accelerate the pitch motion of the flap, in turn increasing the absorption efficiency. From a practical design aspect, the variation of the CF around the peak value is smoother for wider flap, which suggests a broad bandwidth in receiving various wave frequency in irregular waves.