The concept of “average soil skeleton stress” is introduced. The stiffness parameters of unsaturated soils

and

have been derived. The LC yield function

hardening rule and the ellipse yield function of triaxial stress state have been also studied. Based on the physical processes and mechanism of the soil mechanics

the change relationship between the degree of saturation and the volume strain of the soil has been presented by the net mean stress and suction action. The advantage of the constitutive model of unsaturated soils is that it can take into account the important parameters of the degree of saturation in different stress states. The verification results of the partial test data show that the constitutive model is available.