Abstract: To reveal the macro-meso scale instability mechanism of reservoir bank slopes during impoundment, this study takes the Shenjiagou deformed mass in the Baihetan Hydropower Station reservoir area as the research object and analyzes its engineering geological conditions and deformation characteristics. On this basis, a coupled numerical simulation method combining the fluid dynamics and discrete element method (CFD-DEM) is employed to establish a numerical model capable of reflecting the hydro-mechanical coupling effects during the impoundment process. The reliability of the coupled model in simulating the deformation response of the reservoir bank slope is verified by comparing the simulated cumulative slope displacement with field monitoring data. The study systematically analyzes the dynamic evolution of the internal seepage field, particle displacement, and micro-cracks throughout the entire process from initial impoundment to water level rise and subsequent slight drawdown. The results indicate that the rapid water level rise during the initial impoundment stage leads to a significant increase in pore water pressure and a sharp dissipation of matrix suction within the slope, which are the key factors inducing slope deformation and controlling its development. The subsequent slight drawdown in water level does not cause significant deformation aggravation and has a relatively limited impact on slope stability. This study reveals the instability mechanism characterized by water level rise-dominated seepage field evolution, progressive adjustment of mesoscopic particle structures, and eventual coalescence into a macroscopic failure surface, providing a theoretical basis for the stability assessment and early warning of reservoir bank slopes during the impoundment phase.