Abstract: The stress-strain relationship curve of cementitious CSG materials is obtained through the large-scale triaxial tests, and its mechanical properties are analyzed. In response to the obvious nonlinear and softening characteristics of the relationship curve, a segmented description method is used to establish the constitutive relation of cementitious CSG materials. The volume change caused by shear is considered in the curve before failure strength, and a two-modulus K-G model is used to describe it. The mathematical model for the softening curve after the peak strength is obtained through the regression analysis of data and parameter inversion analysis.The parameters of each model can be easily determined through the large-scale triaxial tests.