Abstract: The stability analysis of bedding slopes in soil–rock composite strata under rainfall conditions is complex due to the differing hydro-mechanical properties of rock masses and soil. By developing Fish functions, the saturated–unsaturated seepage module in the FLAC3D software is improved, the calculation of matric suction in the unsaturated zone is refined, and dynamic adjustment of the seepage boundary is achieved. Based on numerical simulations, the influences of the permeability of the underlying rock mass, the dip angle of the soil-rock contact zone, and the rainfall intensity on the stability of bedding slopes in soil-rock composite strata are analyzed. Results indicate that low permeability of the underlying rock mass leads to rainwater accumulation at the soil-rock interface, increasing the susceptibility of the overlying soil to saturation and reducing the slope’s safety factor. A steeper dip angle at the soil-rock contact increases the saturation of the overlying soil. During rainfall, the potential sliding surface first migrates toward the slope surface and then gradually extends into the slope’s interior. With the same total rainfall, lower rainfall intensity promotes more uniform infiltration into the slope, further decreasing the safety factor and deepening the potential sliding surface. This improved method addresses FLAC3D’s limitations in simulating saturated-unsaturated seepage behavior and reveals the evolutionary patterns of slope stability under rainfall conditions, providing methodological support and a theoretical basis for slope reinforcement design.

Key words: soil-rock composite stratum, bedding slope, slope stability, rainfall, FLAC3D