Abstract: After impoundment of the Three Gorges Reservoir, landslides have become one of the most serious geological hazards in this area. These landslides are mainly caused by the declining water levels and rainfall. Taking Hongyanzi landslide in Wushan county as an example, the collapsing initiation mechanism of bank slope combined debris and soil in the areas of Three Gorges Reservoir are studied. Based on the function and algorithm of FLAC^3D seepage analysis module, the mechanism of Hongyanzi landslide is investigated. According to the topographical and geological conditions of Hongyanzi bank slope, the construction of seepage simulation of Hongyanzi landslide is realized by using the finite difference method, and reasonable condition of hydraulic head boundary is confirmed for this model. The change rules of the displacement field are drawn by simulating rainfall infiltration and different water level conditions by writing the built-in FISH function. The stability of bank slope combined debris and soil is analyzed under different water level and rainfall conditions. And the declining stability of bank slope is revealed in the process of rainfall and the declining water levels. The triggering factor of Hongyanzi landslide is the combined effects of rainfall and drawdown of water level. These research results provide a basis for studying the stability of debris and soil bank landslides with different reservoir water level and rainfall conditions.