Abstract: Taking the redbed landslide in Xishui County of Guizhou Province as an example, on the basis of detailed field survey, combined with laboratory mechanical tests and numerical simulation technology, the paper finds out the deformation failure characteristics of landslide and effect of joint fissures and water on the mechanical strength of mudstone, and analyzes the formation mechanism. The results indicate that the landslide is of typical rainfall bedded type caused by the combination of rainfall, terrain, excavation and rock-soil property. Excavation provides free face for shearing of landslide. While the seepage field of slope body changes significantly during rainfall, continuous saturated zone is gradually formed at the interface of strongly and moderately weathered layers, and the change of seepage path caused by excavation results in the formation of large saturated area near the excavated section. The pore water pressure at all monitoring points shows a continuous increasing trend, and its pressure range decreases gradually with the increase of elevation. The development of joint fissures weakens the strength of mudstone. The joint fissures activated by water greatly attenuate the strength of mudstone, with the cohesion decreasing by 82% and internal friction angle by 57.9%. The former is affected more than the latter, and then the slope stability is affected, sliding and shearing along the surface towards excavation direction under the self-weight potential energy, eventually causing severe damage to the houses in front of shear outlet.