Abstract: The genesis mechanism of ancient landslides along reservoir banks is complex, the identification of their stability status is challenging, and their potential hazards are significant. After years of cyclical fluctuations in reservoir water levels, the deformation mechanisms and evolutionary patterns of these landslides have exhibited new characteristics. Taking the ancient landslide in the Tanjiahe area of the Three Gorges Reservoir as a case study, this research employs a multi-source data fusion approach to investigate the current deformation response mechanisms and stability of the landslide under the influences of rainfall and reservoir water level fluctuations. The following conclusions are drawn: The SBAS-InSAR results indicate that the primary deformation in the middle and rear sections of the Tanjiahe landslide is characterized by subsidence, while localized uplift deformation is observed at the front edge. The deformation magnitude in the middle and rear sections is significantly greater than that at the front. The overall deformation velocity ranges from -39.85 mm·a^-1 to 9.36 mm·a^-1,which aligns with the deformation areas and magnitude levels observed in the field. Surface displacement monitoring results show that fluctuations in reservoir water levels and rainfall are key factors driving the persistent creep deformation of the Tanjiayuan landslide. Among these, the rate of reservoir water drawdown has a significant impact on the occurrence of step-type deformation during the drawdown-to-low-water level phase. Inclinometer data reveal the deformation response patterns of the landslide to fluctuations in groundwater levels, providing critical early-warning indicators and reference thresholds for step-type deformation. Additionally, the time-lag effect between shallow surface displacement and deep-seated displacement is clarified. The UDEC numerical simulation results inverted the relationship between groundwater and reservoir water in the Tanjiayuan landslide, including runoff, drainage, and recharge. The study suggests that under extreme conditions, the landslide may experience a deformation failure mode characterized by "horizontal displacement in the landslide-driven block, bulging uplift in the slippage resistance section, and shear failure at the junction between the steep and gentle slopes." The analysis of landslide deformation mechanisms through multi-source data fusion indicates that the Tanjiayuan landslide is currently influenced by the combined effects of "buoyancy weight loss" and "hydrodynamic pressure." During the rise in the reservoir water level, the "buoyancy weight loss" effect is prominent, with its impact decreasing from the front to the middle and rear sections of the landslide. During water drawdown, the hydrodynamic effect is significantly enhanced, and the rate of reservoir water level decline plays a crucial role in the displacement and deformation of the landslide. This study provides an important theoretical foundation and data support for further improving landslide monitoring systems and for a deeper understanding of the deformation and evolution mechanisms of ancient landslides along large reservoir banks.