Abstract: It was indicated by the existing researches that the principal stress ratio in the stress path of rockfill dams kept constant during the period of dam construction.According to the drained triaxial tests under the path of constant stress ratio,a three-modulus incremental nonlinear model was proposed.It could not only provide functional expressions for bulk modulus K and shear modulus G,but also consider the volumetric strain due to the shear stress and the shear strain due to the mean stress with the coupling modulus J.The particle swarm optimization algorithm was used to determine the incremental model parameters through the total constitutive relation curves.Compared with the experimentally measured values,the model could provide good predictions of stress and deformation behavior of rockfill materials.