Abstract: Based on the constitutive framework of unsaturated soils established by the continuum porous medium soil mechanics, an elastoplastic model for unsaturated soils in triaxial stress state is formulated. According to the work expression of unsaturated soils, three stress variables are needed in describing the model: generalized effective stress, modified suction and gas pressure. The gas effect is considered and the constitutive equation considering solid-liquid-gas effect is developed. In the solid phase, a non-associated flow flue is adopted. The plastic change of the degree of saturation and gas strain, representing the influence of the historical evolution of liquid and gas effect respectively, are incorporated in the hardening equations. The soil water characteristic curve is introduced as the liquid constitutive equation, in which the plastic volumetric strain is incorporated to reflect the influence caused by skeleton deformation. Based on the Boyle law and the Henry law, a gas constitutive equation which is affected by the solid and liquid phases is derived. Model predictions are compared with the existing test data, showing that the model agree well with the test data. Comparisons between the model and the one without gas effect are also given.