Abstract: Development of a proper constitutive model is the key to solving the strength and deformation problem in soil mechanics. It is revealed that the conventional Cam-clay model would exhibit unrealistically high strength on the supercritical side and sudden changes in stress-strain relations, and it is incapable of extending the stress tensor to three dimensions at the over-consolidated state. The coupling evolution mechanism in the over-consolidated state is introduced in a state-of-the-art UH constitutive model and the unification of calculation formulas in the over-consolidated and normal-consolidated states based on the elastoplastic theory. Through comparisons with the triaxial compression test results, it is verified that the UH model can satisfactorily describe the stress-strain relations of the over-consolidated soil. The validations against supergravity tests on the vertical behaviour of a circular plate, in terms of the load-deformation curve and lateral pressure coefficient distribution of the soil, demonstrate the significant advantage of the UH model over the Cam-clay model. The essence is that the stress-strain relations of the soil element can be described in a more scientific and rational manner in the UH model. It is well proved that the UH model significantly improves the accuracy and practicability in assessing the strength and deformation problems of the over-consolidated soil, thus capturing important theoretical value and practical significance in solving complex geotechnical engineering problems.