Abstract: The periodic fluctuation of water level in the Three Gorges reservoir area leads to the deterioration of rock mass in the water-level-fluctuation zone, which accelerates the unstable development of some dangerous rock masses in the reservoir area and threatens the Yangtze River waterway with potential surges. This paper takes Guanmuling dangerous rock mass as the research object, and expounds that under the influence of the upper load and the fluctuation of reservoir water, the strength of the weak layer in the lower part of the Guanmuling dangerous rock mass is continuously weakened, and finally evolves into the failure mode of tipping-slip or base fracturing and seat-slip. The stability coefficient of the dangerous rock mass is calculated with the shear strength reduction method under five working conditions. The results show the follows. The dangerous rock mass is basically stable under the current natural working conditions, and the final stability coefficient is 1.3. Under the deterioration condition, the strength is reduced at the rate of 1% per cycle. Under the two conditions of rock mass deterioration and heavy rainfall plus rock mass deterioration, the stability coefficient remains above 1.1 under the influence of 20 hydrological cycles' reservoir water change, which is basically stable. Under the condition of reservoir water and rock mass deterioration, the stability coefficient changes from 1.3 to 1.0, and the stability state also changes from stable state to limit equilibrium state. Reservoir water and heavy precipitation and rock mass deterioration are in the limit equilibrium to unstable state. Under the most unfavorable condition, the displacement of dangerous rock reaches 11 m, the stability coefficient drops to 0.76, and the dangerous rock is in an unstable state. After 20 hydrological cycles of periodic water level change, Guanmuling dangerous rock under rock deterioration condition is in limit equilibrium state or unstable state. It is suggested to carry out special rock deterioration prevention and control to improve the stability of dangerous rock mass and ensure the safety of waterway.