Abstract: Volume changes take place during the fully undrained compression tests on unsaturated soils. It is more difficult to simulate the undrained mechanical behaviors of unsaturated soils than those of saturated soils. In this study, the three-phase coupled governing equations for unsaturated soils under fully undrained conditions are derived using the porous medium theory and the elasto-plastic constitutive model for unsaturated soils. Numerical simulations of the fully undrained triaxial compression tests on unsaturated soils are performed based on the proposed three-phase coupled governing equations. The simulated results demonstrate that the proposed method predicts the hydraulic and mechanical behaviors (shear-induced volume compression, pore water pressure, pore air pressure and degree of saturation increase) of unsaturated soils accurately under the fully undrained conditions. The predictions of the fully undrained triaxial tests on unsaturated soils show good agreement with the test results obtained in literatures, verifying the applicability of the proposed method.