In order to facilitate the calculation of pile-soil interaction in marine engineering, the existing soil constitutive model is simplified taking into consideration the characteristics of marine K0 consolidated soil. On the basis of the modified Cam-Clay model and thermodynamics theory, a unified hardening model for clay and sand is derived combined with the consolidation property of undisturbed soil, using the dissipative work as the hardening parameter. Based on the derived model, the hardening law of K0 consolidated soil under cyclic load is achieved by combining rotation hardening with isotropic hardening. And the shrinkage parameter Θ of yield surface is introduced to describe the evolution of soil yield surface under cyclic load, and then a unified hardening model of marine K0 consolidated soil under cyclic load is established. A comparative study of two cases is carried out to verify the proposed constitutive model. The results show that the calculated value of the new unified hardening model is in agreement with the measured value, and the new model adopts fewer calculation parameters with clear physical significance.