Abstract: As one of the most serious hazards in an earthquake, the collapse is very hard to control largely because the failure mechanism of rock slope is not clearly addressed. Here from the view of Fracture Mechanics and based on the structural characters of natural dangerous rock, the propagation condition of cracks under different seismic waves was analyzed contrastively, and it is found that the tension-shear failure of cracks is the main mode of collapse in an earthquake. Then the energy method was taken as a main way to study the crack propagation direction and rock failure mechanism. The paper proposed approaches to calculate the crack growth amount and critical seismic acceleration, which result in crack failure. Finally, an example was given to validate the theory. The result shows the failure of dangerous rock in earthquake is mainly caused by the discontinuous propagation and continuous accumulation of cracks under tension-shear action of seismic action and along the vertical direction of seismic force.