Xinlong hydropower station is the fourth station of Ganzi-Xinlong segment of cascade hydropower stations in the upstream of the Yalong River. Toppling rock mass develops widely near the dam on the right bank, with a total volume of about 16 million m³. After the reservoir impoundment, the shear strength of the deformation rock is reduced under the effect of groundwater level rise and soaking, leading to the instability of the toppling rock slope and threatening the safety of dam, flood discharge tunnel and water inlet. By analyzing the engineering geological condition and instability mode, we established discreet element model of the toppling rock slope and obtained the deformation characteristics and failure mechanism of the toppling rock. In seismic condition,the deformation failure of slope mainly happens in strong toppling rockmass,which first inclines towards the free face, and finally breaks off. It has features of landslide because shear failure first happens at the slope toe and top and then penetrates through the whole slope. It also has features of collapse in seismic vibration condition. This research could be a scientific basis for the stability evaluation and slope excavation.