Abstract: This paper investigates a large deformation bank slope of a hydropower station. On the basis of researches of geological environment background,this paper investigates the failure modes of the bank slope. The modes include subsiding wedge failure,rock falls,toppling and tension cracking in deep-seated rock mass. The physical simulation test reveals that the failure mode is controlled by multiple structural planes. That is to say,the granite slab-cracked rock mass is toppled along the structural surfaces dipping into the slope face,and is stretched and sinked along the structural surfaces dipping out of the slope face. The toppling causes the root of the massive rock mass to break. The fracture surface traces the moderately inclined structural surface. The toppling and breaking trace steeply and gently inclined structural surfaces,and gradually form a macroscopic continuous fracture surface. The deep deformation boundary of the rock mass is a typical stepped shape. Moreover,on the basis of geological analysis and physical simulation of bank slope deformation and failure modes,the formation mechanism of large deformation of slope is demonstrated. This paper concludes that the genetic mechanism of large deformation of unstable slope is a combination of the upper pushing deformation and the lower traction deformation. The geomechanical model is summarized as that block toppling-slip composite deformation failure mode.