Abstract: Stratified loess exhibits characteristics of transverse isotropy, often exists in an unsaturated state, and demonstrates significant elastoplastic deformation. However, constitutive models that simultaneously account for these three aspects—transverse isotropy, unsaturation, and elastoplastic behavior—have not been reported. To address this gap, a comprehensive study is conducted through systematic laboratory tests and theoretical analysis to investigate the elastoplastic constitutive model and mechanical properties of transversely isotropic unsaturated loess. First, considering the transversely isotropic effects of matric suction, an elasticity constitutive equation for transversely isotropic unsaturated loess is developed. The elastic component is described using an elasticity constitutive equation for transversely isotropic unsaturated soil, while the plastic component is characterized by a yield function and potential function based on the generalized Mohr-Coulomb criterion, along with a non-associated flow rule and a strain hardening rule. Second, a series of triaxial tests under different stress paths are designed to determine the elastic and plastic parameters of the model. Finally, the model is preliminarily validated through unsaturated true triaxial tests under various intermediate principal stresses and net confining pressures. The experimental results show good agreement with the model predictions. This research provides accurate insights into the strength and deformation characteristics of transversely isotropic unsaturated loess under complex stress paths. It offers theoretical support and a scientific basis for deformation and stability analysis, as well as engineering design of natural stratified foundations and large-scale filled foundations. Furthermore, it contributes to the enrichment and development of constitutive models in soil mechanics.