In rough sea conditions, semi-submersible platform often suffers from extreme wave impact loads, which can result in structural damage. It is important to predict the wave impact loads on semi-submersible platform. Therefore, the purpose of this study is to investigate the wave impact loads on semisubmersible platform with numerical methods. A numerical method, based on a fixed regular Cartesian grid system, has been developed by the authors. In the method, the FDM (Finite Difference Method) is applied for solving flow field, and the THINC/SW (Tangent of Hyperbola for INterface Capturing with Slope Weighting) model, which is kind of VOF (Volumeof- Fluid) model, is adopted to capture the free surface. Some selected model test cases, form Exwave JIP project, will be used to validate the present numerical method and to analyze the wave impact loads on semi-submersible platform.