A large number of landslides are induced by rainfall, posing a great threat to the resident life and wealth security. Rainfall infiltration is a key factor causing slope instability. To study the mechanism of rainfall infiltration affecting the stability of slope, a hydraulic-mechanical coupling numerical experiment was conducted to reveal multi-field evolution in the slope during the coupling process and its impact on slope stability. The results indicate that as the coupling process progresses, the pore water pressure in the slope gradually increases, decreasing the effective stress. This will cause deterioration in shear strength, leading to the damage on the rock-soil mass gradually and triggering the slope failure.