The deep overburden in coastal soft soil areas will magnify ground motion significantly, which will have adverse effects on engineering construction. Based on the frequency domain analysis method of seismic response in one-dimensional horizontal soil layer, inputting different seismic waves, the equivalent linearized dynamic calculation model is used to analyze the shallow site seismic motion characteristics of different site conditions under 7-degree seismic fortification. The calculation results show that the maximum acceleration, the maximum shear stress, the maximum shear strain and relative displacement of the shallow soil layer within 70 m can be significantly affected by the difference of shallow soil layer thickness or seismic wave spectrum. The variation of relative displacement with depth does not conform to the characteristics of cosine function recommended by the code. The cosine curve apparently underestimates the rate of change in displacement (shear strain) at the shallower position and overestimates the rate of change in displacement (shear strain) at the deeper position. In this paper, a fitting relation is proposed to estimate the relative displacement of shallow soil layers within 70 m in coastal soft soil areas under 7-degree seismic fortification. This fitting relation has certain limitations, but can provide reference for seismic response analysis of underground engineering in coastal areas.