Abstract: Self-developed geotechnical three-dimensional swelling permeameter is used to explore swelling property of bentonite-quartz sand mixtures as buffer material in different dry density (1.6 g/cm³, 1.7 g/cm³, 1.8 g/cm³ and 1.9 g/cm³) and mixed sand ratio (0%, 10%, 20% and 30%).The test results show that the time-history curve of swelling pressure has experienced the rapid swelling stage, the slow expansion stage and the stable expansion stage at the temperature of 30 ℃.The rapid swelling stage occurred during the saturation process at about 350 min, basically reaching 80% of the ultimate swelling pressure, and the specimen attained equilibrium swelling pressure within 1 200 min.The swelling pressure of bentonite-sand mixtures has anisotropy.The ratio of the horizontal swelling pressure and the vertical swelling pressure is about 0.9, and the horizontal swelling pressure is smaller than the vertical swelling pressure.The three-dimensional swelling pressure increases exponentially with the dry density, and decreases exponentially with the sand content.When the sand contents increased from 0% to 30%, the vertical swelling pressure decreased from 4.62 MPa to 1.23 MPa.When the dry density increased from 1.6 g/cm³ to 1.9 g/cm³, the vertical swelling pressure increased from 0.8 MPa to 3.1 MPa.The test results are important for the study of expansive properties of Gaomiaozi bentonite.