Abstract: A proposed reservoir dam stands in a V-shaped valley. The ground is composed with the semi-longitudinal brick red sandstone and foliated or lenticular mudstone, silty sand mudstone intermediate layer. It has no evident fault structure. It has an old landslide. The rain is plenty. Based on investigation, the old landside has landslide debris accumulation, slipped surface and the bottom disturbed zone. The development of vertical and horizontal structure planes of the landslide area and bedding plane constitute a wedge-shaped slot. The cause of the landslide formation can be due to the fact that the slippage-tension failure took place at the wedge-shaped slot under heavy rain. The limit equilibrium method is used to calculate the stability factors. The stability factors are 1.45 in the natural state, 1.39 in the seismic state and 1.27 in the heavy rain state, respectively. Therefore, the landslide is stable in the natural state. The role of earthquake impact in the stability is not significant. But the stability of the landslide will be lower under the heavy rain. Finally, the stress-strain field and deformation failure features of the ancient landslide at this stage are reproduced through finite element simulation.