Abstract: Rainfall infiltration is one of the important factors triggering the instability of slopes. Accurately assessing its impact on slope stability has significant engineering implications. However, existing research mainly focuses on two-dimensional model analysis or neglects the weakening effect of soil strength during the rainfall infiltration process, making it difficult to fully reveal the instability mechanism of rainfall-induced landslides. Based on the Mein-Larson infiltration model and smoothed particle hydrodynamics (SPH) method, a three-dimensional model of rainfall-induced landslides was established to analyze the evolution process of the slip surface at different infiltration depths, quantitatively assessing the impact of rainfall infiltration on slope stability and systematically studying the evolution characteristics of slope stability under rainfall infiltration conditions. The results show that: (1) the safety factor of the slope is negatively correlated with infiltration depth; as the infiltration depth increases, the stability of the slope significantly decreases; (2) due to the differences in soil properties on both sides of the wetting front, the slip surface in the longitudinal profile evolves into an asymmetric composite form, while in the transverse profile, the displacement area evolves into two intersecting semi-elliptical displacement zones; (3) the instability of rainfall-induced landslides is a gradual process where the surface saturated soil first experiences localized instability, which then triggers deep soil deformation through stress redistribution, ultimately forming a continuous slip surface leading to overall instability.