Intense earthquake shaking inevitably ruptures the rockmass along the fissures and bursts mountainous slopes, and these understable slopes are namely cracked slopes. For such slopes, aftershocks, rainwater infiltration and others factors will inevitably induce crack propagation and the crack thus gradually accumulates to be the cutting boundary that controls the slope deformation and failure. To understand how the slope stability varies in the process of cracking expanding, upper bound approach is employed to assess the cracked slope in Wenchuan earthquake epicenter considering the effects of crack propagation, rainwater infiltrating and earthquake. The results conducted in this paper indicate that the crack propagation lowly reduces the slope stability and it does not directly destabilize the slope. But the crack length owing to propagation inevitably increases the probability of slope failure, which more depend on the filling water in the crack and the earthquake intensity.