Abstract: The wetting deformation behavior of unsaturated loess is studied under the framework of unsaturated soil mechanics.Based on the principle of effective stress of unsaturated soil is expressed by intergranular attraction, a constitutive model for wettig deformation of unsaturated loess is established.Through theoretical analysis, the integral expression for the yielding function of loading-wetting and the explicit formula for collapsibility coefficient of loess are given.Combined with the experimental data of undisturbed unsaturated loess, the characteristics of collapsing deformation under different hydraulic paths are analyzed, and the variation law of collapsibility coefficient is discussed.The results show that the model can well reveal the mechanical mechanism of deformation of unsaturated collapsible loess, and can provide some reference value for the researches on wetting deformation properties of loess and engineering application.