Abstract: For the investigation of spatial variation of seismic motion, the wave theory and superposition principle are used to illustrate the spatial variation of motions caused by seismic waves. And plane SV wave’s incidence in semi-infinite space is used as a case in this paper. Firstly the difference in superposition pattern of waves in different position is investigated and the space region is divided into different domains according to superposition pattern. Then the border line between superposition domain and separation domain with different Poisson’s ratio and SV wave’s incident angle is discussed. Finally the difference in peak value and duration time of particle motion in different domains is investigated. The result shows that, the border line is usually controlled by z₃ (separation line generated by reflected P wave and reflected SV wave). Particularly, there exist two special cases, one is the vertical incidence of SV wave in which the border line is controlled by z₁ (separation line generated by incident SV wave and reflected SV wave), the other is the zero amplitude of reflected SV wave in which the border line is controlled by z₂ (separation line generated by incident SV wave and reflected P wave). The time history of different particle motion in one depth has the same shape with the influence of incident and reflected waves. In time history of particle motion, the period contributed by three waves only exists in depth lower than z₁ and the duration time decreases with depth. The period contributed by two waves exists in all superposition domain and the duration time increases firstly and then decreases with depth. The period contributed by one single wave increases with depth and reaches its maximum in separation domain. The increase of particle motion duration has two slope change in depths of z₂ and z₃. The peak value of particle motion changes dramatically in superposition domain near free surface and remains constant in deep superposition domain and separation domain.