Abstract: A modified SFG model is developed based on the principle of effective stress. With the effective stress of unsaturated soils deduced from the expression of input work, the degree of saturation is naturally introduced into the proposed model without any additional parameters. After a simple introduction to the SFG model, the volume-change behaviors and the soil-water characteristics are discussed, and then the loading collapse curve is formulated. The loading collapse yield equation in the modified model is a function not only of suction, but also of the current degree of saturation. The yield surface provides a smooth transition between saturated and unsaturated states, which is convenient for simulating the plastic deformation during drying process. The simulated results of the modified model are compared with the experimental data available in literatures, showing that the modified model agrees well with the experimental results. Compared with the original SFG model, the new model yields better results at different degrees of saturation.