Abstract: The long-term stability of reservoir bank slopes is a critical issue for reservoir safe operation.Focusing on a reservoir bank slope at a hydropower station in southwestern China, this study systematically investigates the multi-factor coupled mechanisms governing slope deformation, with particular emphasis on the controlling role of creep deterioration effects.Based on the Nishihara mechanical model and multivariate regression analysis, a prediction method for long-term slope deformation is proposed, which comprehensively considers rock mass softening, long-term creep, and effective stress variations.Quantitative predictions of future deformation trends are also provided.The results indicate that periodic fluctuations in reservoir water levels, especially during the drawdown phase, are the dominant factor inducing rock deterioration in the fluctuation zone and driving long-term slope deformation.Specifically, the viscoplastic deformation of rock in the fluctuation zone exhibits stepwise growth under 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 water-level fluctuation zone are key measures to ensure the long-term stability of reservoir bank slopes.This study provides references for long-term deformation prediction and stability evaluation of reservoir bank slopes.