Abstract: Choosing an appropriate content of quartz sand is a key issue for the study of sand-bentonite mixtures used as buffer/backfill materials. It can increase the thermal conductivity and strength of the mixtures while meeting the basic requirements of low permeability and well swelling pressure simultaneously. Based on the instantaneous profile theory, the unsaturated hydraulic permeability of densely compacted sand-Gaomiaozi bentonite (3:7) mixtures with a dry density of 1.9 g/cm3 is tested under unconfined conditions. The results show that the measured unsaturated hydraulic conductivity is between 2.1×10-15 and 1.0×10-12 m/s, and increases with the decrease of the suction but changes gently. Compared with the coefficient of unsaturated hydraulic conductivity of Gaomiaozi bentonite under unconfined conditions, the measured unsaturated hydraulic conductivity of mixtures shows slight increase. Mixing with sand improves the thermal conductivity of the mixture, and keeps the requirement of low permeability for buffer/backfill materials.