Abstract: Based on linear visco-elastic assumption, the precision of results influenced by the scope of soil region is analyzed when seismic excitations are traveling waves. In frequency domain, the seismic response of soil layer varying with artificial boundary, damp coefficient, exciting frequency, the ratio of width and depth of soil layer, and the apparent velocity of earthquake wave is discussed. At the same time, the characteristics of horizontal and vertical seismic response are analyzed. Based on above conclusion, the seismic response of a river-crossing topography is calculated. The results show that the scope of soil layer is used to determine the parameters of soil satisfy aetory.