Abstract: For soils, the variations of the mean stress and the deviatoric stress induce plastic volume strain and plastic shearing strain. Based on the dilatancy equation, a coupling stress is proposed considering the influences of both the mean stress and the deviatoric stress on plastic strain. With the isotropic compression and swelling regularities of clay, referring to the necessary and sufficient conditions of path independency for curvilinear integration, a one-dimensional constitutive model describing the relationship between the plastic volume strain and the coupling stress is deduced. By using this method, an elasto-plastic constitutive relationship for sand is also established. From a new visual angle, a method for understanding and establishing the soil constitutive model is introduced by deducing the coupling stress and one-dimensional constitutive relation.