To investigate the variation of lateral pressure on the backside of retaining walls in different rainfall conditions and the influence of expanded polystyrene (EPS) layer modulus on stress-strain behavior of rigid-flexible composite walls, this study employed GeoStudio and FLAC3D software to numerically simulate the behavior of embankments with expansive soil subjected to rainfall. The results indicate the following: ① With rainfall infiltration, the lateral pressure on the backside of the wall significantly increases; during the rain cessation period, the lateral pressure increases for deeper soil layers, while it decreases for the surface layer. ② Compared to rigid walls, the rigid-flexible composite walls exhibit significantly reduced lateral soil pressure and horizontal displacement; as the modulus of the EPS cushion decreases, the compression of the cushion increases, leading to a significant improvement in pressure reduction. ③ The modulus of the EPS layer should be selected based on the specific engineering requirements.