Abstract: Based on the experimental results and previous studies, the stress ratio at the failure (M_f) under different confining pressures is modified, and the pressure dependency of plastic modulus is discussed. Considering the idea that the hardening rule of Lade-Duncan single yield surface model is defined by introducing plastic work to yield surface expression, the plastic modulus is modified using the plastic work within the framework of the generalized plasticity theory, and a modified P-Z model is proposed. To check the validity of the improved method, the modified approach is applied to the constitutive model by Liu et al., and the modified P-Z model is employed to simulate the other two triaxial tests on rockfill materials. Satisfactory agreement between the numerical and experimental results under different confining pressures confirms the capability of the modified model in improving the prediction of stress-strain behaviour of rockfill materials.