In order to evaluate the safety of the repository (disposal container, buffer material and surrounding rock), an unsaturated Thermo-Hydro-Mechanical (THM) fully coupled numerical model was built to conduct the unsaturated multifield-coupled numerical simulation. Based on the Comsol Multiphysics platform, the high-level radioactive waste repository was simulated for 500 years, with the basis of experimental data from the candidate area of the Beishan high-level radioactive waste repository in China and the Gaomiaozi bentonite. Results of the coupled simulations were compared with those of uncoupled simulations, and the parametric sensitivities of permeability, heat transfer coefficient and underground water depth were analyzed. The results show that temperature in coupled simulation is 13 degrees Celsius higher than that in uncoupled simulation. When the water table rises from 0 m to 1,000 m, the time it takes to fully saturate the buffer material will fall from 703 to 28 years. The saturation time of the buffer material will increase from 28 years to 250 years when the permeability of the surrounding rock decreases by five orders of magnitude. Each physical field is coupled to each other, which affects the long-term stability of the repository.