Abstract: Toppling failure is one of deformation and failure types of bedded rock mass with high dip angle.It usually develops near the ground surface.It is thought that only small-scale toppling deformation can be retained completely before.Owing to the special topographic feature and structure of rock mass,a large-scale intact toppling deformation body was found at present at Xiaowan Hydropower Station left bank slope front dam.The toppling deformation body is near to the damsite and shows higher terrain.Once the toppling deformation failures happen the loss will be disastrous for the construction of the Hydropower Station.The size and structure of the rock mass was found out so as to offer evidences for choice of support projects and evaluation of the slope stability.Based on the field investigation,we discovered that a large-scale toppling deformation occurred at the damsite,which was developed to a great depth of about 200m.In this paper,the deformation phenomena revealed by surface excavation and underground galleries were described in detail including variation of the dip angle and rock mass structure.In terms of these,a mechanical model of the slope deformation is established and its formation process is analyzed.