Objective: The YMM landslide is the largest landslide accumulation body nearest to the dam in the near-dam reservoir section of the Three Gorges Reservoir area. The landslide, located on the north bank of the Yangtze River within Zigui County, Yichang City, Hubei Province, with a surface area of approximately 0.48 km² and has a total volume of approximately 2 000 × 10⁴ m³. It is situated 17 km upstream of the Three Gorges Dam. Owing to its massive scale, sensitive location, and severe consequences of potential instability, the YMM landslide has attracted significant attention. Nearly 20 years of observation data indicate that although the landslide's deformation has been slow, it has continued without convergence. Methods: This study comprehensively considers the relationships among geological conditions, external influencing factors, and deformation characteristics of the landslide. A stepwise linear regression method is applied to analyze the observational data. Combined with a mechanical model of the hydrodynamic triggering mechanism of reservoir bank landslide deformation, the study quantitatively decomposes the roles and effects of various external triggering factors in the landslide's deformation process. Based on the phase-transition time nodes of these effects, the deformation evolution process due to reservoir impoundment is divided into three stages. Results: The study shows that the YMM landslide was stable before the impoundment. The reservoir impoundment led to its reactivation, which was followed by a complex deformation adjustment process. In the first stage (June 2003—September 2006), the landslide was reactivated in a retrogressive mode by a significant rise in the reservoir water level. In the second stage (October 2006—September 2018), the deformation mode shifted from front retrogressive to overall creep deformation, mainly due to the deterioration of the landslide rock-soil medium caused by reservoir water infiltration. The deformation rate gradually decreased as the deterioration effect weakened, and reservoir water level fluctuations had a more significant influence than seasonal rainfall during this period. In the third stage (October 2018—February 2024), the deterioration process of the physical and mechanical properties of the rock-soil medium induced by water-rock interaction was essentially complete. The landslide adapted to changes in the groundwater environment, resulting in a further significant reduction in the overall deformation rate. During this stage, the influence of seasonal rainfall on landslide deformation exceeded that of reservoir water level fluctuations. In terms of geological conditions, landslide characteristics, and deformation patterns, time-dependent deformation mainly convergent creep indicates that the landslide is generally stable. However, extreme rainfall remains a key triggering factor for potential local instability of the YMM landslide. Conclusions: This study provides a robust framework for interpreting the long-term deformation evolution of large-scale reservoir landslides by integrating monitoring data, statistical modeling, and mechanical analysis. Identifying stage-specific deformation patterns and dominant triggers enhances the understanding of landslide behavior in response to external forcing. These insights are crucial for improving early warning systems and developing targeted mitigation strategies in similarly high-risk reservoir environments.