Abstract: The impoundment of the Three Gorges Reservoir triggers a large number of reactivated and deformed ancient landslides along the reservoir banks.The concept of “ductile deformation” is introduced to explain the differential deformation response of ancient landslides under similar influencing factors.Taking the Sanmendong Landslide in the Three Gorges Reservoir area as an example, we analyzes the deformation characteristics and the mechanism of “ductile deformation” during the reactivation process based on field geological investigations and 17 a of monitoring data.The research findings indicate that:(1) The Sanmendong Landslide is a gentle ancient sedimentary landslide that is reactivated and deformed after the impounding of the Three Gorges Reservoir.The main deformation features at the front, middle, and rear parts of the landslide are bank collapse, tensile cracks or slumping, and tensile-shear cracks respectively.(2) During the falling stage of reservoir water level, the landslide deformation is sensitive to rapid water level decrease.When the reservoir operates at low water levels and during the water rising periods, rainfall especially continuous heavy rainfall is the main triggering factor of landslide deformation, resulting in sustained deformation.(3) During the occurrence of “ductile deformation”,the internal part of the landslide undergoes a process of compaction, leading to increased soil density and enhanced cohesion, thus improving the shear resistance of the soil and providing the strength to resist subsequent deformations for a certain period.The research results can provide a reference for landslide monitoring and early warning in Three Gorges Reservoir area.