Abstract: A new expression form of coupled disturbing variable for unsaturated soils is defined in this paper,in which both of disturbance factors that external force and wetting are reflected.Moreover,a new constitutive model of unsaturated natural soils is build up in accordance with homogenization theory for composites materials.In the model,the coupling of hydraulic hysteresis and stress-strain behavior in unsaturated natural soils and the influence of saturation change on soil structure degradation are considered,and the evolution equation of coupled disturbing variable is presented.Comparing the calculation results of the new model with the experimental results,it can be seen clearly that the new model is satisfactory in describing the influence of soil structure degradation on the mechanical properties of unsaturated natural soils that is strain-softening behaviour.