Abstract: Rainfall infiltration is one of the key factors triggering slope instability. Accurately assessing its impact on slope stability is of great engineering implications. However, existing studies mostly focus on two-dimensional models or overlook the weakening effect of soil strength during rainfall infiltration, making it difficult to comprehensively reveal the instability mechanisms of rainfall-induced landslides. In this study, based on the Mein-Larson infiltration model and the smoothed particle hydrodynamics (SPH) method, a three-dimensional model of rainfall-induced landslides was developed to analyze the evolution process of slip surfaces at different infiltration depths. The study quantitatively evaluates the impact of rainfall infiltration on slope stability and systematically investigates the evolution characteristics of slope stability under rainfall infiltration conditions. The results indicate that: (1) the slope safety factor negatively correlates with infiltration depth; as infiltration depth increases, slope stability significantly decreases; (2) due to differences in soil properties on either side of the wetting front, the slip surface in the longitudinal profile evolves into an asymmetric composite shape, while in the transverse profile, the displacement zone develops into two intersecting semi-elliptical displacement zones; (3) the instability process of rainfall-induced landslides is gradual, beginning with localized instability in the saturated surface soil, followed by deep soil deformation caused by stress redistribution, ultimately leading to the formation of a continuous slip surface and overall slope failure.