<p>The soil structure of slopes formed during deep excavation is highly sensitive, and extreme rainfall events can easily trigger landslides, posing serious risks to construction safety and causing significant economic losses. The potential hazards of such events have attracted considerable attention in the academic community. Clarifying the failure mechanisms of slopes under rainfall is crucial for ensuring the stability of excavation slopes and preventing landslide disasters. This study combines laboratory experiments and numerical simulations to investigate the evolution of soil mechanical properties, soil–water characteristics, and overall slope stability of clay slopes under rainfall. The results indicate that soil cohesion gradually decreases with increasing water content, following a Boltzmann function. Once the water content exceeds a threshold, cohesion declines rapidly. Variations in water content caused by rainfall infiltration are a major factor leading to slope failure. The infiltration process shows that as infiltration depth increases, the infiltration rate decreases; higher rainfall intensity results in faster soil response, and longer rainfall duration leads to deeper infiltration effects. The failure mode of cohesive soil slopes typically begins with surface erosion, which deteriorates the soil structure, followed by local collapses and gully formation, eventually leading to large-scale deformation and failure.</p>