Abstract: For the plane strain problems of, such as lateral deformation of wall soil and lateral extrusion deformation of fill embankment foundation induced by excavation of foundation pits, with the help of the transformational loading system of the true triaxial apparatus in Xi'an University of Technology, the vertical loading of loess embankment under the plane strain and the lateral unloading of loess foundation pit are simulated. The experiments of vertical loading and lateral unloading are made to study the stress-strain relation of loading and unloading loess with different moistures under plane strain. It is revealed that the relationships between the major and minor principal stress difference and the axial strain of original Q₃ loess in the vertical loading stress path under plane strain, as well as the axial strain and the lateral strain are hyperbolic. Those between the major and minor principal stress difference and the lateral strain of original Q₃ loess in the lateral unloading stress path as well as the lateral strain and the axial strain are also hyperbosic. Based on the trifarious tangent elasticity relation under the plane strain, the relation between the tangent elasticity modulus and Poisson’s ratio under change of stress-strain condition is derived, and a non-linear constitutive model is established. By comparing the calculated and test results of the stress path of loading and unloading of intact loess under plane strain, two kinds of non-linear models in the path of both plane strain are proved to be right.