Abstract: In the deep geological repository for high-level radioactive waste, the thermal conductivity of buffer/backfill materials is significant for safe operation. For the Gaomiaozi（GMZ） bentonite buffer/backfill materials, the evolution of thermal conductivity under coupling of thermal-chemical conditions and technological void effects was investigated via the thermal probe method, to analyze the influence of different dry densities, temperatures, and salt concentrations; combined with dry density, moisture content, and microstructure tests, the mechanism was clarified. Results show that the thermal conductivity of the joint area in the bentonite blocks is lower than that of the block area. The thermal conductivity increases with dry density and environmental temperature, but decreases with salt solution concentration. It is inferred that the dry density and water content of the saturated bentonite blocks are still heterogeneous perpendicular to the sealed joint after 150-day hydration. The joint area is weak with poor thermodynamic performance. When the environmental temperature rises, the latent heat transfer of water vapor is promoted, so the thermal conductivity increases. When the salt solution concentration increases, the large pores decrease and the small pores increase in samples. It means the attenuation of the swelling of diffuse double layers and the latent heat transfer of water vapor, offsetting the effect of temperature on thermal conductivity and resulting in the final decrease.