Abstract: A large accumulation body was identified in the reservoir area of a hydropower station located on the upper reaches of the Lancang River, which could undergo deformation and potential instability after reservoir impoundment. To assess this risk, a detailed field investigation was conducted, followed by a three-dimensional physical model test to study the failure mechanism and disaster evolution process of the accumulation body under fluctuating water levels. Numerical simulations were also performed for comparison and validation. The results indicate that deformation of the accumulation body is closely correlated with changes in water level. During water level rise, the deformation progresses through two stages: creep deformation and subsequent deformation attenuation, showing an initial increase followed by a gradual decrease. Instability is most likely to occur when the water level drops between elevations 2810 m and 2831 m. During the impoundment period, deformation alters the original structure of the accumulation body, and prolonged water immersion weakens the soil, creating conditions favorable for slope failure. A rapid decline in water level leads to outward seepage within the slope, which serves as the direct trigger for landslide instability. The findings provide a basis for further research on surge disasters induced by the failure of such accumulation bodies, and offer valuable insights for addressing similar engineering challenges.