Abstract: There are many unstable anti-dip slopes in the Gongjiafang-Dulong section of the Three Gorges Reservoir area. These slopes are generally over 500 m high, and usually possess thin-thick interbedded and soft-hard interbedded structures. Field investigation show that D7 slope has obvious S-shaped flexible deformation. In order to further explore the mechanism of S-type toppling, this paper adopts the methods combining investigation, statistics, GPS monitoring and DEM simulation. Through field investigation and UAV tilt photography, a three-dimensional model of Dulong slope is established. And then through extracting the aerial data, the statistical laws of broken length and layer thickness are obtained. Based on the statistical laws, the S-type toppling mechanism of D7 slope under the action of reservoir water softening is simulated by the discrete element software of UDEC. The results show that:(1)Dulong slope has a steep anti-dipped layered structure, and three sets of structural planes cut the rock mass into blocks. (2)According to the proportion of soft rock, the Dulong slope can be divided into two types. The soft layer number of A-type slope account for about 20%, larger than that of B-type slopes with a proportion of 10%. The ratio of broken thickness and layer thickness(S/T) of A-type slopes is situated between 0.7 and 2.8 and concentrates on 1.2~1.8. The ratio of B-type slopes situates between 0.5 and 3.3 and concentrates on 0.6~1.5. (3)Long-term monitoring shows that the slope moving is larger when the reservoir water level dropping, and the continuously increasing displacement of the D7 slope makes it possible to lose integral stability. (4)The DEM simulation shows that the fracture surface extends from the slope toe, and the hard rock layers in the middle slope restrain the deformation of the upper part. The differential deformation between hard and soft rock layers and the inertia difference in different parts of deformable body are the main causes of S-type toppling. Two shear belts form across the two inversion points of S-shaped layers respectively, and the slope finally form flexible bending-slip failure. (5)The ratio of S/T can affect the slope failure mode. The larger the ratio is, the smaller the reverse deformation area and the toe collapse area are. When the S/T≥2, the slope failure mode turns to flexural toppling without S-shaped deformation.