Seismic actions in the head region of the Three Gorges Reservoir(TGR) have increased significantly since the beginning of dam impoundment. Coupled with cyclical water level fluctuation, the deformation mechanism of landslides beside TGR are worthy of research. A typical accumulative landslide located in Xingshan County was selected as a case study in this paper. Three typical adverse conditions were determined by monitoring relationship between the water level fluctuation and time of seismic occurrence. Then, using a seismic intensity of V degree, a multi-field coupling numerical simulation was conducted to determine the deformation mechanism of the landslide. The results of the simulation afford the following conclusions:(1) Seismic action is closely related to water fluctuation, while superposition of adverse flow conditions and seismic load, the coupling effect of the seepage field and dynamic field causes a landslide deformation.(2) The structure of the debris landslide is relatively homogeneous, except for the region close to the free face of body of debris. There are no significant undesirable geologic elements and seismic amplification is not reflected in the debris. Furthermore, the mechanical parameters of debris and bedrock are quite different, thus the boundary of debris and bedrock is also a deformation border.(3) When the seismic intensity attains the V degree, a deformation landslide caused by continual earthquakes is limited and convergent, but long-term deformation monitoring for landslides is necessary.