Based on the deep cutting landslide treatment project in Caixi Township, PLAXIS 3D was used to establish a slope model with complex cracks under rainfall conditions, and the existence of cracks and the slope deformation stability under different fissure depths were analyzed. The result shows that the sliding surface of the slope changes significantly with increasing fissure depth. When the crack depth is close to the potential sliding surface, the crack divides the landslide into two separate parts, one is the crack surface itself and the other is the original sliding surface below the crack. This segmentation effect leads in a significant reduction in the slope stability. It is further found that there is a threshold effect between the crack depth and the slope stability, that is, the slope stability does not change much before the crack depth reaches a certain critical value. When the crack depth exceeds this critical value, the slope stability will decrease sharply. It reveals the influence mechanism of fissure on slope stability, in order to provide a scientific basis for slope engineering design, disaster prevention and treatment.