Based on the static liquefaction of granular materials, an elastoplastic static liquefaction constitutive model was proposed in the framework of critical state soil mechanics. The yielding surface and hardening rules were obtained by integrating the contact force of the model proposed by Chang and a non-associate flow rule was adopted as well. The model has taken the state dependent dialatency law and the effect of initial density to the stress-strain relationship into consideration. The parameters of the model are simple and have certain physical meanings. The predicted results obtained by the model have a good agreement with the undrained triaxial test of Toyoura sand and sand-silt mixture.