In order to investigate in-depth the impact of water level fluctuation on the stability of slopes with multiple accumulated deposits, this article takes the example of the deep accumulated deposits in Longwangxi. Based on the exploration of engineering geological conditions and the surrounding areas affected by historical landslides, a large-scale 3D numerical model is established to calculate the stability, deformation, and plastic zone variations of different accumulated deposits on the slope under different water level conditions through fluid-soil coupling analysis. The influence of water level changes on the stability of accumulated deposits is analyzed based on the numerical results. The analysis shows that when the water level is below the range of accumulated deposits, the impact of water level fluctuation on their stability can be neglected. However, as the water level rises above the range of accumulated deposits, significant deformations occur at the front edge of the deposits due to the softening and suspension-induced reduction in weight of the soil-rock mass, leading to even larger overall deformations of the accumulated deposits. In extreme cases, such as when the upstream reservoir gates of tributaries are opened, resulting in a sudden drop in water level, the entire slope of the accumulated deposits may become unstable. Furthermore, accumulated deposits located above the highest water level line may experience significant displacement due to disturbances caused by the instability of deposits below, thus indirectly affected by water level fluctuations.