The Luozhuang Group 3 landslide in Xiaoling Town, Zhashui County, is the focus of the present study.Geotechnical tests are performed indoors based on the geological survey in the field. By means of large-scale direct shear test and numerical simulation, the shear mechanical properties of sliding zone soil under different water content conditions are analyzed and studied. On this basis, the formation mechanism and deformation process of rainfall-induced accumulation landslide are revealed. The large-scale direct shear test shows: in the early stage of the test, the shear stress and shear displacement are linear, and the curve shows nonlinear characteristics with the increase of shear displacement. When the other test conditions are the same, the shear stress is negatively correlated with the water content. When the water content is the same, the shear stress increases with the increase of the normal load. The numerical simulation results show that the accumulation layer slope is prone to instability under the action of rainfall factors, with the continuous increase of rainfall intensity and duration, the soil moisture content increases or is prone to damage after saturation, and the slope stability coefficient decreases continuously. Under the condition of rainfall, the deformation of the landslide body of the accumulation layer is relatively strong, and its deformation process generally undergoes three stages, creep deformation-accelerating deformation-sliding failure. The form of failure movement is mainly push-type, which has a destructive effect on the threat object through the pushing effect, resulting in personnel and property losses. The research results have certain reference significance for the mechanism research and disaster prevention and mitigation of colluvial landslides in Qinba Mountain area.