Abstract: Beishan area in Gansu Province has been chosen as one of the candidate sites for China’s HLW repository, and Gaomiaozi(GMZ) bentonite was identified as the most promising backfill material. Due to the intrusion of ground water, a potential problem was taken into account for the long-term safety assessment of the repository, clay colloids generated from the eroded bentonite might play an important role on radionuclide transport. In the present work, the stability of colloids prepared from the bentonite, which was sourced from Ⅲ ledge in Gaomiaozi region, was investigated by the photon correlation spectroscopy. Furthermore, batch experiments were conducted to get a better understanding of the effects of variable pH and background electrolyte concentration on the U(Ⅵ) adsorption onto GMZ bentonite colloids. The results show that under acidic condition, the Kd values of uranium increase with the pH increase and reach a peak maximum value in the range of pH≈7. Besides, under alkaline condition, the Kd values of uranium decrease with the pH increase. The concentration of background electrolyte NaClO₄ in the selected range of this study has little effect on the adsorption of U(Ⅵ) on Gaomiaozi bentonite colloid.