Abstract: The Three Gorges Reservoir area presents a complex geological environment, where the deterioration of rock masses in the riparian zone of karst bank slopes is expedited by the fluctuating reservoir water levels of the reservoir, thereby hastening the evolution of the bank slopes towards instability. This study focuses on the Huangyanwo dangerous rock mass in the Three Gorges Reservoir area. A comprehensive filed investigation was conducted to scrutinize the deterioration phenomenon of rock masses within the water–level fluctuation zone, and a numerical analysis was performed to assess the long–term stability of the dangerous rock mass. The findings reveal the presence of a vertical karst zone and a bottom seepage zone in the Huangyanwo dangerous rock mass. The bottom seepage zone is situated within the water–level–fluctuation zone, characterized by areas of weakness and rock mass deterioration. Considering the influence of reservoir water levels and rainfall events, the stability coefficient of karst water pressure on the bank slope is calculated to be 1.69, indicating that the dangerous rock mass remains stable. Nevertheless, due to the progressive deterioration of rock mass parameters within the weak area at the bottom, the stability coefficient experiences an annual decline of approximately 0.01. It is predicted that the Huangyanwo dangerous rock mass will transition to an unstable state after approximately 57 cycles of periodic water level variations, and the instability failure will occur after 62 cycles. The failure mode of the dangerous rock mass involves a compound failure mechanism of toppling and sliding subsequent to the connection of the weak zone at the bottom, aligning closely with the qualitative understanding grained from field investigations. The research results hold significant instructive implications for the early warning and prevention strategies concerning analogous geological disasters in reservoir areas.