For safety assessment of a high-level radioactive waste disposal system, it is important to predict and analyze the coupled thermo-hydro-mechanical (THM) behaviors of bentonite, which is a buffer candidate material in the engineered barrier system. The Barcelona Basic Model (BBM) is a constitutive model to describe the geomechanical behaviors of partially saturated soils. Complicated tests are required to directly measure BBM parameters of bentonite. In this study, we demonstrate that probable BBM parameters can be sought by calibrating the BBM parameters to match simulation results to observed ones for two kinds of simple tests (swelling pressure test and free swelling test) instead of the complicated direct tests. In the swelling pressure test and free swelling test that were conducted by Japan Atomic Energy Agency (JAEA), water was injected into constrained and unconstrained bentonite core samples, and then swelling pressure and displacements were measured, respectively. We find optimal BBM parameters using a quasi-Newton optimization method that reproduce the observed swelling pressures and displacements in hydro-mechanical simulations. The optimal BBM parameters that are sought in the inversion process can be used to predict the THM behaviors of bentonite barriers in a high-level radioactive waste disposal system.