On the basis of geological process analysis in the topple failure of rocky slope, provided that the equilibrium state and interlamination resistance which prevent shearing offset damage do always exist between upper and lower edges of stratum, the stress of some point on rock mass can be simplified to vertical self-weight and horizontal lateral stress. In this way, the formula of the critical depth for topple failure can be deduced by turning the geological and mechanical process into plane strain model and adopting maximum stress failure criterion. Living example and discussion show that the critical depth theory about toppling failure applied widely, conform to actual failure，and also take horizontal stress into account . Besides, the slope and stratum dip angle as well as the thickness have greater influence on rock toppling failure, especial the slope of stratum angle between 50° and 70° is the most likely to topple.