Abstract: In order to understand the migration behavior of ¹²⁹I in the host rock of China’s key potential deep geological repository, and to provide basic data for the pre-safety assessment of the site, the effect of different experimental conditions on the adsorption and diffusion of iodine ion was investigated using a batch sorption method and a modified capillary in-diffusion method. The results suggest that the adsorption of iodide onto Beishan granite is almost negligible under solid/liquid ratio of as high as 20 g/L. According to the capillary experimental results, D_a values of ¹²⁵I^- are between 8.2×10^-11 and 1.4×10^-9 m²/s, which suggests that the diffusion rate of iodide ion in compacted Beishan granite powder is very high. There is a positive correlation between the D_a with temperature, isotopic carrier concentration and ionic strength, which can be explained by Brownian motion and electrical double layer structure of granite surface. While D_a value of ¹²⁵I^- witness an overall decrease with the increase of pH values due to the granite surface charge. As can be seen, the capillary method is a fast and easy method which can be used to investigate the migration of radionuclide in compacted mineral powders.