Abstract: Among the triggering factors of post-earthquake rock landslide, the rainfall water plays a most important role. However, different from that of soil slope, the mechanism of its effect on rock landslide failure is not clear. From the view of fracture mechanics and based on the characters of post-earthquake rock, the mechanism of crack propagation, the way of water infiltration and the development process of sliding face are studied. By using the upper bound theorem, the influence of fissure water on the stability of rock slope is quantitatively studied. Finally, an example is given to prove the theory. The results show that the propagation and coalescence of fissures and the softening of sliding face are the main reason for the failure of post-earthquake rock landslide under rainfall.