Abstract: According to the deformation properties and geological conditions at field, the factors affect the toppling deformation of rock mass are analyzed. Contrasted to the real deformation strength, which exposed by excavated slope, the numerical simulation method called UDEC is used to simulate the toppling strength along the limit depth. The factors, time-dependent process and distribution are researched. The research result is that the external factors are gravity force, earth stress, unloading direction and magnitude. A calculating model of limited toppling depth according to the shallow deformation strength is put forward based on geological mechanism analysis, numerical method and formula reduction.