Abstract: The spatial variability of materials is a common problem in geotechnical engineering. For the coarse-grained soil, the essence of its spatial variability is the uneven distribution of particle gradation and density. However, the classical state-related theory can only describe the influences of density and stress level, which are only two state variables. The development of a three state variables-related constitutive model that considers gradation, density and stress level represents a significant advancement in the field of coarse-grained soil engineering. The introduction of a gradation parameter enables the quantitative characterization of changes in the gradation curve of coarse-grained soil. This has led to the proposal of the critical state equation and isotropic consolidation equation, which considers the effects of gradation, density and stress level. Furthermore, a three-state variables-related constitutive model for coarse-grained soil is established based on the principles of the generalized plasticity theory. The proposed model employs a single set of model parameters, which are capable of accurately representing the stress-strain characteristics of coarse-grained soils under diverse gradations, densities and confining pressure conditions. Furthermore, it can be uesd in the numerical analysis of coarse-grained soil engineering, taking into account the spatial variability of materials.