Based on the results of triaxial experiments on rockfill materials, a dilatancy equation is derived by considering the nonlinear dependence of the dilatancy stress ratio on the stress ratio (i.e. the ratio of the deviatoric stress to the mean stress). This equation reveals the deformation characteristics of rockfill materials induced by dilatancy appearing at low confining pressure and by shear shrinkage appearing at high confining pressure. Within the framework of the generalized plasticity theory, the plastic flow direction vector and the loading direction vector are deduced. A compression index is determined in terms of compactness-dependent and mean stress-dependent effects. Then the plastic modulus is formulated as a function of the mean stress, the shear stress ratio and the compactness. On the basis of above studies, a new elastoplastic constitutive model is derived by considering the particle breakage of rockfill materials. The methods are addressed to determine ten material parameters of this model. Finally, the proposed model is used to simulate the triaxial tests on rockfill materials under different confining pressures and stress-paths, and the simulation results are good agreement with the experimental data.