Abstract: The long-term stability of reservoir bank slopes is a critical issue for reservoir safety. This study focuses on a reservoir bank slope at a hydropower station in southwestern China, systematically investigating the coupled mechanisms governing slope deformation, with particular emphasis on the controlling role of creep degradation effects. Based on the Nishihara model and multivariate regression analysis, a prediction method for long-term slope deformation is proposed, which comprehensively considers rock softening, long-term creep, and effective stress variations. Quantitative predictions of future deformation trends are also provided. The results indicate that the periodic fluctuations of reservoir water levels, especially during the drawdown phase, are the dominant factor inducing rock degradation in the fluctuation zone and long-term slope deformation. Specifically, the viscoplastic deformation of rock in the fluctuation zone exhibits step-like growth with cyclic water-level changes, gradually becoming the primary contributor to cumulative slope displacement. Therefore, controlling and dynamically monitoring the viscoplastic deformation of rock in the fluctuation zone are key measures to ensure the long-term stability of reservoir bank slopes. This study provides theoretical and methodological support for the prediction of long-term deformation and stability evaluation of reservoir bank slopes.