In order to ensure the long-term reliability of the turbine, the wave conditions should be considered when designing the tidal turbine. A three-bladed Horizontal Axis Tidal Stream Turbine (HATST) with a diameter of 0.9 m is used as the benchmark model, and the open source Computational Fluid Dynamics (CFD) library OpenFOAM is used to predict the performance of HATST under wave and current conditions. The grid dependence tests are conducted to select the optimal grid number to capture the flow characteristics. As a verification study, the generated wave profile is compared with the results obtained from third-order Stokes wave theory, and it is found that the simulation results are consistent with the theoretical results. The effects of different wave frequencies and heights on the power performance of HATST are studied, and the results indicate that the wave height is of more significant impact on the power performance.