Because of broken structure caused in earthquake, the dangerous rocks collapse frequently under the action of special external force after the event, which restricts the reconstruction greatly. Here from the real structure of rock, based on the analysis of destructive external force, and according to the fracture mechanics, the failure mechanism of dangerous rocks is studied clearly by modeling reasonably. Then the condition of different kind’s collapses is given and proposed to diagnose dangerous rock. At last, an example is given to prove the theory. The result shows the aftershock, rainfall and their coupling action are the main forces that cause the collapse after the event, and there is a critical minimum depth of crack under any action which can lead the failure. Meanwhile it is also found that the critical depth is smaller under couplings action.
