Abstract: The nearshore seabed often has a certain slope, which has a large impact on the characteristics of seafloor ground motion. Based on the analysis of the relationship between seabed slope and ground motion characteristics, an attempt is made to propose a critical value of slope affecting offshore ground motion. This provides a theoretical reference for whether or not to consider sloping site factors in the seismic design of ocean structures. The seismic fluctuation analysis model of different slope submarine sites is established by combining the self-programmed seismic fluctuation program and the finite element dynamic analysis software ADINA. And the effect of slope on offshore ground motion is analyzed by taking the vertically incident pulse P wave and SV wave as the seismic load. It is found that the sloped site has a greater influence on the P-wave component, but the variation of ground motion characteristics decreases when the slope is greater than 5%. On this basis, the calculation range of seabed slope is reduced, and the range of slope critical value that affects the characteristics of offshore ground motion is proposed, that is, the critical slope. The influence of factors such as water depth and site conditions on critical slope is further discussed. It is found that there is a critical value of 1%~2% slope for the slope site effect affecting the characteristics of offshore ground motion. And the surface cover of the seabed has some influences on this critical slope.