Abstract: In large reservoirs, the bank slopes experience the effect of water level variation for a long time. This effect mainly contains two aspects: 1) the influence of hydraulic pressure going up and down; 2) the water-rock degradation effect of rock mass in hydro-fluctuation belt. Based on the above and the previous studies and considering the depth and time of water-rock effect, the strength degradation model for rock mass is established and used to analyze a typical bank slope. The study shows that water level variation has distinct influence on the stability of bank slope. Especially after a period of time, the safety factor decreases dramatically and regains quickly during high water level operating period. It is indicated that the balance of sliding force and anti-sliding force in the bank slope is broken and then comes to a new balance. With the increasing times of water-rock effect, the dramatic decrease of safety factor appears forward during a cycle time of a year, and the frequency and decrease amplitude both increase. Furthermore, the greater the degree of water-rock interaction is, the more easily the balance of rock mass is broken and the worse the stability of bank slope is. The safety factor of bank slope in the new balance decreases year by year. This phenomenon conforms to the damage accumulation process of gravity anticline type landslide during the period of reservoir water fluctuation. This study is of a certain guiding significance to the stability calculation of bank slopes in reservoirs.