Abstract: The strength deformation characteristics of loess are complicated.It is important for engineering construction to accurately predict the mechanical behavior of loess.The constitutive model for loess is studied.The unified strength theory is introduced.A unified yield criterion in terms of stress invariants is determined.The strength constituent of loess is identified.The evolution of cohesion strength and friction strength is explained during the deformation.Combining with the basic theory of binary medium model, the models for the cementation and friction mediums of loess are established, as well as the damage parameters.Based on the principle of equivalent strain, a unified binary medium constitutive model for loess is proposed.The determination of model parameters is analyzed.The constitutive model is verified by the triaxial tests on loess.The comparison between the proposed model and the unified elastoplastic model is conducted.The applicability of the binary-medium model is further analyzed.