Abstract: The progressive failure of landslide is a process from quantitative change to qualitative change. It usually starts from the foot of the slope and continues to destroy along a weak surface until a through sliding zone is formed. In order to study the influence of peak strength and residual strength on landslide instability, a strain softening model is adopted for sliding zone soil. Firstly, we use two-dimensional examples to illustrate the variation of sliding surface morphology, plastic zone and displacement of landslide, and invert the progressive failure process of landslide in two-dimensional space. Then we divide the instability process of the landslide into three stages:the creeping stage, the pressing stage and the sliding stage according to the displacement change. Based on the theory of slip-soil soil strain softening, the author studied the progressive failure process and failure law of landslide in three-dimensional space. The results show that in the three-dimensional space, the landslide first forms a plastic zone at the foot of the slope and the perimeter of the landslide. After that, the plastic zone increases along the perimeter of the landslide and forms a penetrating plastic zone. Comparing the two-dimensional and three-dimensional examples, it shows that the three-dimensional strain softening model of sliding zone soil is more realistic to reflect the whole process of progressive failure of landslide. Therefore, the strain softening model of sliding zone soil can truly reflect the progressive failure process of slope, which provides a theoretical basis for the early warning and monitoring of landslide.