Local strong ground motion can be significantly affected by site conditions. The characteristics of low-velocity surficial sediments play an important role in local site effects. Previous numerical studies and analyses of observation data have suggested that the amplification of strong ground motion is related to thickness and the properties of low-velocity sediments. However, these analyses have mainly been based on one-dimensional-layered sedimentary models. In this study, we use a two-dimensional lateral heterogeneous model to study the effects of low-velocity surficial sediments on strong ground motion. The constructed numerical model includes the seismic source, an elliptical sedimentary basin, and a low-velocity surface sedimentary layer above the basin. We simulated seismic wave propagation in this model using a hybrid finite difference and pseudospectral method on staggered grids. The results show that multiple reflections and conversions occurring within the low-velocity sedimentary layer enhance the duration of ground motion, and their constructive interference significantly enlarges the peak ground amplitude. The most significant amplification of the peak amplitude occurs near the edges of the basin. The horizontal peak ground amplitude shows a stronger amplification than the vertical component inside the basin. With increasing wave velocity in the surficial layer, the energy of multiple reflections and conversions decreases within the sedimentary layer. The duration of the ground motion decreases as well. However, the amplification of strong ground motion shows similar patterns for models with different thicknesses and seismic velocities. The study results show the mechanism of ground motion amplification by the low-velocity surficial layer as a process of seismic wave propagation. This suggests that the amplification mechanism of the surface sedimentary layer on the seismic ground motion for models that include the source, sedimentary basin, and surface layer is more complicated than that from a one-dimensional-layered model. The ground motion amplification is a joint effect of the sedimentary properties (thickness and seismic wave velocity), lateral heterogeneity of the sediments, seismic source, and seismic wave propagation path. Therefore, the studies of strong ground motion must consider the comprehensive effect of these factors.