Abstract: The soil-rock mixture is a special geological material between discrete and continuous media. Due to the influences of genesis, component type and content, its mechanical behavior is complex and changeable, which may lead to the excessive deformation and shear failure of high fill projects of soil-rock mixture. Therefore, it is of great theoretical significance and engineering application value to study the deformation and strength characteristics of soil-rock mixture and its constitutive models. Based on the protection embankment project of Xiangbiling resettlement site in the reservoir area of Baihetan Hydropower Station, the consolidation and drainage shear tests on two kinds of fill materials of soil-rock mixture under three different confining pressures are carried out by using the large-scale triaxial test device, and the variation laws of deviatoric stress and volumetric strain with axial strain and the characteristics of shear contraction and dilation of soil-rock mixture are analyzed. On this basis, according to the critical state and bounding surface elastoplastic theories of granular soil, the critical cohesion and state parameter suitable for the soil-rock mixture are introduced, and based on the spatially randomly distributed micro-shear structures, a state-dependent three-dimensional multi-shear bounding surface model for the soil-rock mixture is established to decompose the complex macro-deformation of the soil-rock mixture into a macro-volume deformation and a series of spatially distributed and mutually independent one-dimensional micro shear deformation. The simulated results of the model are compared with the triaxial shear test ones, which verifies that the proposed model can reasonably describe the strain softening and shear dilation characteristics of the soil-rock mixture under low confining pressure and the strain hardening and shear contraction characteristics under high confining pressure.