Abstract：Steep and antithetic platy structure of rock mass and geomorphology of the longitudinal valley in the dam site area cause wide-range toppling deformation on the cross-strait slope. Aimed at solving problems of toppling deformation, the authors, in the light of regional engineering geological conditions, analyzed characteristics of toppling rock mass deformation, made degree classification of toppling ruptures, established discrete element slope models for excavation and failure mechanism, and detected characteristics of the excavated slope and four developing processes of failure mechanism. There exist shear dislocation in weak toppling rock deformation, tensile deformation on layers in strong toppling rock deformation, strain-shear rupture through layers in strong toppling rock deformation and tensile fracture on very strong toppling rock rupture. The results provide a scientific basis for stability evaluation of the dip-slip body and the excavation work.