To overcome the limitations of the conventional super-subloading surface constitutive model in capturing soil stress-induced anisotropy and the nonlinear degradation of shear modulus at small strains, this study presents an enhanced model. By incorporating the g(θ) method and integrating classical small-strain stiffness theory, the proposed model offers improved representation of both anisotropic behavior and the nonlinear shear modulus reduction under small-strain conditions. The new model was subsequently applied to predict triaxial shear test results for Shanghai soft clay, Fukakusa clay, and Hefei slightly expansive clay, as well as to simulate the deep excavation of the Huifu Road Station in Hefei Metro. The results demonstrate that the proposed model effectively captures the high initial shear modulus and its nonlinear attenuation under small-strain conditions, unifies the application of four yield criteria (von Mises, Mohr-Coulomb, Matsuoka-Nakai, and Lade-Duncan) to characterize stress-induced anisotropy, and successfully describes both the structural shear-shrinkage softening of soft clay and the shear-dilation softening of overconsolidated soil. The improved model not only effectively characterizes complex mechanical behaviors of natural soil but also accurately predicts deformation patterns of retaining piles during excavation processes.