Abstract: In the frame of the Theory of Mixtures, the paper proposed and developed a new method for the constitutive modelling of saturated and unsaturated porous media. In the new model, realistic porous media is viewed as three kinds of mixtures within two levels. In order to simulate the degradation and softening properties of geomaterials, an elasto plastic damage model is also proposed, which, in addition, can also take into account of the influence of unsaturation on the overall properties of geomaterials. As an example, finite element analysis is performed to simulate the deformation and strength properties of a conventional triaxial specimen.