Abstract: In order to explore the intricate mechanical properties of the cemented sand and gravel (CSG), large-scale triaxial shear tests are conducted on the specimens with varying confining pressures and gel contents. A bimodal shear strength criterion and a double-yield surface constitutive model applicable to the CSG are established. The main conclusions are as follows: (1) The CSG exhibits a certain level of cohesive strength and structural integrity, demonstrating notable mechanical characteristics such as softening and pronounced shear dilatancy at the macroscopic scale. (2) The shear strength criterion, based on the binary medium theory, is developed to describe the evolution of strength in the CSG under varying gel contents. (3) The shear strength criterion is appropriately transformed into the shear yield surface within the constitutive model, and by considering the tensile resistance of materials a volumetric yield surface is established based on the modified Cam-clay model. Additionally, the constitutive model emphasizes to describe the strain softening and strong dilatancy of the CSG. (4) The stiffness matrix of the constitutive model is derived for the general stress states, and its excellent fitting with the triaxial shear tests on the CSG is demonstrated. The findings of this study can provide better theoretical guidance for stress-deformation calculations in CSG dams.