Deformation instability of slope due to rain was one of the most common geological disasters. Speaking from the mechanism, the destructive action and triggering action of rainfall affected the stability of slope itself, which made the slope deform and even lead to instability. June 28, 2010, Guanling in Guizhou province a large landslide occurred which triggered by continuous heavy rainfall, it caused that in two village-groups 107 villagers were buried, all the tragedy of houses were completely destroyed. Using finite element software ANSYS to simulate the mechanical behavior of the landslide, to analyze the deformation mechanism, we got that the large landslide deformation mechanism was a complex mechanical geological process. From the analysis of interaction between soil and rock, the landslide showed complex mechanical geological features. As rainfall increased the strength of unsaturated soil and weak intercalation reduced, this strengthened the role of the slope deformation and failure. The lock-fixed section first appeared brittle fracture, then that brought shear failure to the creeping section. The interaction between lock-fixed section and creeping section was the key of slope failure.