According to a typical example of semisubmersible platform, the air gap of platform has been studied through the numerical model tuned and validated by the tank and wind tunnel test results with ANSYSAQWA. The wind, wave and current loads are simulated simultaneously. Meanwhile, the effect of the full scale mooring system has been simulated. In the 300, 400 and 500m water depth, the sensitivity of wave slamming with respect to water depth has been studied based on the Det Norske Veritas (DNV) specification in 45°, 90° and 180° load directions through 9 cases and their 90 subcases. The 90% Gumble distributions for the 10 subcases of each case are chosen as the final results of the air gap. It is found that the results are obviously sensitive for the air gap and wave slamming through the same environment condition, mooring line type and pretension. Therefore, the water depth should be considered for the analysis of air gap and wave slamming in the design progress for engineering project.