Abstract: Taking the engineering site in the offshore area of China as the research object, the typical calculation model of saturated submarine free field is constructed after fully considered the influence of self-weight stress of overlying sea water, and the dynamic finite difference method is implemented to carry out two-dimensional seismic response analysis. In this paper, we discussed the influence of the thickness of overlying sea water on the peak ground motion value and response spectrum of seafloor under the condition that SV and P waves of different amplitudes are used as the input of the basement, and systematically analyzed the causes of the differences. The results show that when SV wave is input into the sea floor, the peak acceleration of the sea floor surface is smaller than that of the overlying anhydrous site surface, and the effect of the thickness of the sea water layer on the peak acceleration can be ignored; when P wave is input into the sea floor, the peak acceleration of the sea floor surface is larger than that of the overlying anhydrous site surface, and the peak acceleration of the sea floor surface gradually decreases with the thickness of the sea water layer increase.