Abstract: In order to accurately predict the long-term operation behaviour of concrete-faced rockfill dams of pumped-storage power stations, the unified generalized plasticity constitutive model for coarse-grained dam materials is modified by introducing a modulus factor with a positive second derivative, which can improve the model’s behaviours in simulating the “hysteresis loop” during the unloading-reloading process and reflecting the modulus’ s gradually decrease during unloading. Meanwhile, the model’s effective scope of application is extended to the post-peak softening stage through incorporating functions related to the stress ratio and the plastic shear strain. Based on the laboratory triaxial experimental data, the model’s effectiveness is validated. Results show that the modified model can better reflect the “hysteresis effect”, “Ratchet effect” and softening of the coarse-grained materials. In addition, the explicit expressions of the yield surface under the model parameter β equal to unity are supplemented, which can be used for finite element numerical analysis of long-term behaviour prediction of pumped-storage rockfill dams.