The accumulation body is extensively distributed in the valley area of major rivers in southwest China, and its stability directly impacts the safety and stability of dams during rainfall conditions. To study the geological evolution and disaster processes of landslides during rainfall events, a centrifugal model test system is designed to simulate heavy rainfall on the slope of a reservoir bank, with a focus on investigating the induced instability of the accumulation body at full storage capacity. Following the test results, additional numerical simulation is carried out to analyze the influence of rainfall intensity, reservoir level state and permeability of accumulation body on slope stability. The results indicate that during the reservoir storage stage, the slope front of the accumulation body experiences reduced resistance to sliding due to buoyancy and water softening. However, osmotic pressure exerted on the slope body also contributes to its strengthening. The cracks formed at the rear edge as a result of these two competing mechanisms significantly influence the development of slope deformation during the rainfall stage. Rainfall primarily leads to surface erosion and runoff on the slope, with a small portion infiltrating through cracks, resulting in shallow subsidence of the rear margin. If the slope is left untreated, it is likely to experience overall failure. Therefore, addressing the cracks at the rear edge of the bank slope is crucial for preventing and controlling geological hazards.