Abstract: Based on the critical state theory, a constitutive model was proposed for structural loose sand. The structural parameter was used to revise the yield function, thereby predicting the plastic deformation of loose sand more appropriately. Another parameter was used to adjust the hardening parameter and dilatancy equation, thus giving a better description of contraction behavior for loose sand. The introduction of the parameters emphasized the dependency on both density and effective principal stress of sand. Linear isotropic consolidation and critical state lines were applied in void ratio versus the power of effective mean stress space, and more accurately reflected the stressstrain behavior of sand. A comparison between model simulations and experimental results showed the validity of the proposed model.