Abstract: In the design of a high-level radioactive waste repository, the buffer material is generally backfilled around the waste canister in the form of bentonite block splicing, and the gaps between the tank body, the block and the surrounding rock are filled with the joint material. Based on the experimental data of GMZ01 bentonite and the basic theory of thermo-hydro-mechanical coupling (THM) in porous media, a THM coupling calculation model was established, numerical calculations are performed on the Comsol Multiphysics platform. The effects of joints on the expansion, the permeability and thermal conductivity of buffer materials, and the self-healing effects were studied. The types, the influence of quantities, dry density and width of joints, as well as the splicing forms of blocks on the permeability were analyzed. The joints can greatly increase the permeability of the buffer material, but the permeability of the joints decreases significantly over time. The final average permeability of the material mainly depends on the average dry density, while the distribution of the joints and the splicing of the block are less affected.