In order to understand the influence of physical, chemical factors and temperature on the maximum swelling pressure and give the relative expression equations, the physical, chemical and mechanical tests in laboratory are carried out for the buffer material made of domestic bentonite, water and sand. The water retention curves and the water diffusivity curves of the buffer material are obtained; the empirical formulae are established for the relationships between unsaturated swelling pressure and maximum swelling pressure, saturation degree and suction as well as saturation degree and temperature and rock pressure; the control equations for solving coupled thermo-hydro-mechanical processes in unsaturated area, especially considering swelling stress, are derived, and the relative elasto-plastic 2D and 3D FEM codes are made up. For validating rightness of the mechanical models and the codes the numerical simulations are conducted against the laboratory tests and the in-situ tests. This study can develop and deepen the understanding for mechanism of coupled thermo-hydro-mechanical phenomena, and rich and improve the relevant theories, and help designing and constructing nuclear waste disposal projects more safely, reliably and economically. This has an important practical meaning for the nuclear industry departments of China. At the same time, the research results acquired have also appropriate reference values for the study of coupled thermo-hydro-mechanical phenomena in the fields of oil recovery in deep stratum, geothermal utilization, probing mechanism of earthquake production and volcano eruption and so on.