The stress-strain relation of undisturbed loess with varied initial moisture content was examined via conventional triaxial shear test and isotropic compression test. Shear test revealed that the critical state lines of undisturbed loess approximated to be straight lines which shifted along the q axis up and down horizontally with the variation of moisture content but do not pass through the coordinate origin of the p-q plane. Triaxial compression test demonstrated that the e-lnp curve of undisturbed loess was a broken line rather than a straight line, and the spherical stress at the fold point declined with the increase of initial moisture content. Under the hypothesis of ellipse yield locus, the initial yield function of undisturbed loess was acquired, according to which the initial yield stress and initial moisture content in different stress paths can be predicted. The present research lays a foundation for establishing the constitutive model of undisturbed loess during the process of moistening deformation.