Abstract: During the Tangshan earthquake sand boiled extensively and serious damages were occurred at Xiji district, Tongxian, Beijing. This place was an abnormally high seismic intensity area. There were no liquefaction and severe damage in the adjacent region. In this paper, the ground motion is discussed by the dynamic analysis at two typical liquefied and unliquefied sites. Under cyclic loadings the stress-strain relationship of soil is represented by Iwan model. The solution of the motion equation is given by the explicit finite difference method, and the half-wave method is developed for calculating the water pressure produced due to cyclic loading. The results obtained in this paper are, to a large extent, consistent with the macroscopic phenomena observed in these sites after the earthquake.