Through a large-scale triaxial test, a new type of nonlinear K-G-D constitutive model, which can describe the strain softening and dilatancy of cemented sand and gravel (CSG) material, was developed based on the K-G constitutive model. The bulk modulus, shear modulus, dilatancy modulus, and other important parameters of the model were determined according to the test results and regression analysis. The nonlinear K-G-D constitutive model was verified. The results show that the calculated values agreed with experimental data, indicating that the model can provide references for the design and analysis of dams with CSG materials.