Abstract: Reservoir water fluctuation can often trigger deformation and failure of reservoir bank landslides. This study focuses on the Hongyanzi landslide in the Pubugou hydropower Station of the Dadu River to investigate the deformation characteristics and deformation mechanisms of the reservoir bank landslides. Through detailed surface macro deformation surveys, monitoring data analysis, and GeoStudio numerical simulation, the deformation characteristics, seepage field, stability and the influence of reservoir water on landslide were studied in depth. The research revealed significant macro deformation of the Hongyanzi landslide surface, with a cumulative displacement curve exhibiting a “step-like” characteristic. The primary inducing factor of landslide deformation was found to be the decrease in the reservoir water level. When the reservoir water level dropped from 850 m to below 830 m, the displacement step was triggered, and the cumulative deformation of the “step” segment accounted for over 90% of the total annual deformation. Accelerated deformation of the landslide occurred when the rate of the reservoir water level decline was greater than 0.5 m/d. The landslide deformation mode was identified as creep slip-tensile cracking, with the rise and fall of the reservoir water level significantly impacting the internal permeability of the sliding body and causing a large impact on the landslide’s stability, leading to the “step-like” deformation of the landslide.