Setting a flexible pad cusion behind the rigid retaining wall can effectively reduce the lateral earth pressure. Expanded polystyrene (EPS) geofoam is a common flexible material that has the characteristic of easy compression. In this paper, through the retaining wall model test in laboratory, the lateral earth pressure distribution of different EPS boards with different elastic modulus and thickness is studied; the influence of elastic modulus and thickness on the decompression performance of EPS boards under different loadings on the filling surface is analyzed in detail. The influence of compression value of EPS board on the lateral earth pressure on retaining wall is also analyzed by FLAC^3D numerical modeling. The research results show that the EPS board can effectively reduce the lateral earth pressure behind the retaining wall; the smaller the elastic modulus of the EPS board, and the larger the thickness, as well as the external load on the surface of the backfill wall, the decompression effect of the EPS board becomes more significant. After the EPS board thickness increases to a certain extent, the decompression effect of the EPS board will not increase significantly. The best decompression effect can be obtained when the EPS board thickness reaches 0.1 times the wall height.