Abstract: Based on the unified hardening parameter, an extended critical state constitutive model for sand is proposed. One state parameter is used to adjust the hardening parameter and dilatancy equation, giving a better description of dilatancy and contraction behavior for sand. The other parameter is used to revise the yield function, predicting the plastic deformation more appropriately. The introduction of the parameters into the model emphasizes the dependency on both the density and the effective principal stress of sand. The linear isotropic consolidation and the critical state lines are applied in void ratio versus the power of the effective mean stress space. The combined effect of the density and the effective mean stress on sand can be described by the proposed model using a unique set of model parameters in a wide range of density and confining pressure. A comparison between model simulations and experimental results shows the validity of the proposed model.