Abstract: To describe well the stress-stain behaviour of sands, generally, a state parameter is used. It is based on the critical state void to established the elastoplastic constitutive relations. However, it is difficult to obtain the critical state parameter in triaxial drained tests for dense sands. The phase transformation state is a characteristic state. Its related parameters are easy to measure. So this paper defines the state parameter that is based on the phase transformation state void. It introduces the stress ration expressions for dilatancy and boundary, and establishes the elastoplastic constitutive model for sands. Finally, based on the provided model parameters, the simulated results fit well with the test results and realistically reflect the strain hardening and softening characteristics for dense sands.