Abstract To reveal the evolution law of the mechanical failure of the root-soil composite and identify the main control factors and their coupling and mutual feeding relationship, this paper takes the most common naturally growing plants in Yan ‘an area as the research object and studies the evolution process of the mechanical deformation and failure of the root-soil composite by applying the methods of in-situ pull-out test, indoor direct shear test of the root-soil composite, numerical simulation, and theoretical analysis. The mechanical characteristics of root-soil interaction were analyzed, and the mechanism of root-soil fixation was explained. The results show that: (1) the root-soil composite’s mechanical deformation and failure characteristics have obvious regularity and stages and are affected by plant growth state, root morphology, soil physical and mechanical properties, and other factors. (2) There are obvious evolutionary stages in the deformation and failure process of the root-soil composite, that is, the coordinated deformation stage of the root-soil, the stress redistribution stage, the secondary root break stage, the main root break stage and the complete failure stage, which correspond to the linear deformation section, the acceleration section, the shock rise section, the steep fall section and the residual deformation section of the F-S curve (Force-displacement curve)obtained by the in-situ pull out test. (3) In the in-situ pull-out test, the final failure body of the root-soil composite was inverted cone shape. The root fracture interface was basically near the boundary of the final inverted cone failure body, in which the stress state of the root system was directly affected by the stress-strain state of the microelement and the characteristics of the root material. (4) The plant roots showed obvious oblique deformation and axial tensile stress with the soil shear dislocation on the fracture surface, which verified the rationality of the “oblique root” hypothesis based on the transformation of shear stress to tensile stress.