Abstract: The historical activity of the Xianshuihe Fault has resulted in the development of numerous large-giant landslide deposits along both banks of the Xianshuihe River. Reservoir impoundment associated with hydropower development has profoundly altered the hydrological conditions of these deposits. To elucidate the reactivation mechanisms of landslide deposits under conditions of a large-scale rise in high water level, the Jiaowu landslide deposit in the Xianshuihe Reservoir area of the Lianghekou Hydropower Station was selected as a representative case. Comprehensive methods including detailed engineering geological investigation, unmanned aerial vehicle (UAV) photogrammetry, satellite remote sensing interpretation, and numerical simulation were employed to identify the spatial structural characteristics and failure mechanisms of the Jiaowu deposit. The results indicate that: ① The deformation characteristics of the Jiaowu landslide deposit exhibit pronounced spatial and temporal heterogeneity. Geological conditions constitute the fundamental cause of landslide deposit formation, rock mass structure serves as the controlling factor, and the large-scale rise in high water level acts as the key triggering factor for deposit reactivation. ② The rare 80.00 m water level difference between the dead level and the normal level in the Lianghekou Reservoir primarily induces extensive inundation and softening to the frontal slope, leading to a significant reduction in the shear strength of the toe materials and subsequent instability. The resulting loss of frontal support generates a free face, which further triggers gravitational sliding of the rear part of the deposit. ③ Integrated analysis suggests that the deformation mode of the Jiaowu landslide deposit under conditions of a large-scale rise in high water level is characterized by a sliding failure along the bedrock-overburden interface, controlled by frontal traction and rear gravitational driving. This process can be divided into three stages: frontal deformation, frontal traction-rear driving, and reactivated sliding. The primary reactivation mechanism involves long-term reservoir water immersion and erosion of the frontal materials under high water levels, which cause structural degradation of the slope toe soils (including local loosening, progressive expansion, and tensile cracking). This progressive loss of support at the front ultimately leads to overall slope instability and sliding.