A new compound model is developed to calculate volumetric strains of unsaturated and saturated collapsible loess. In this model

loess is regarded as the compound of undisturbed and disturbed parts

both of which are respectively and uniformly distributed in loess; and the proportion of the two parts can be calculated through the combination of a pore size distribution curve; a suggested geometric structural model and structural instability criterion with the generalized suction being considered. The volumetric strain increment includes three components: the elastic increment of undisturbed part; the plastic increment of disturbed part determined by the application of the generalized suction theory; and the structure collapse contribution which is particularly relevant to the change of void ratio from undisturbed soils to disturbed soils. The relationship between the angle-stiffness of the structural instability criterion and the degree of saturation is determined by the structural strengths revealed by compression tests of natural loess. The rationality of this model is verified by comparing calculated results with published experimental data.