Abstract: At present, deep geological disposal is one of the feasible schemes for highlevel radioactive waste disposal. Tamusu clay rock is one of the alternative clay materials for surrounding rock of highlevel radioactive waste repository in China. Tamusu clay is inevitably exposed to radiation in the repository, which may affect the microstructure and composition of geological materials such as clay, and further affect the migration behavior of nuclides in geological media. 99Tc (t1/2=2.1×105 a) is the key radionuclide for safety assessment of highlevel radioactive waste geological disposal. However, the research on 99Tc diffusion in Tamusu clay is relatively little. Therefore, the following experiments were carried out in this paper. Firstly, the clay was irradiated by 60Co, and the chemical composition and physicochemical properties of the clay before and after irradiation were tested. The results show that the main components of the clay are clinoptilolite, quartz, zeolite and dolomite, and the main components of the clay before and after irradiation are basically unchanged. However, the specific surface area of the irradiated clay increases and the pore diameter decreases. The content of organic carbon in clay decreases after irradiation, which may affect the diffusion rate and adsorption amount of nuclides. Secondly, the effects of compaction density, ionic strength and pH on the diffusion behavior of Re(Ⅶ) (instead of 99Tc(Ⅶ)) in Tamusu clay by capillary method were studied. The results show that the order of magnitude of diffusion coefficient of Re(Ⅶ) in Tamusu clay before and after irradiation is 10-10 m/s. The apparent/effective diffusion coefficient (Da/De) decreases with the increasing density. The apparent diffusion coefficient conforms to the onedimensional diffusion model for density. And the De values of Re(Ⅶ) could be related to the porosity by Archie’s law. While the cementation factor is 0.8 and 2.1 respectively. The Da and De values of Re(Ⅶ) increase with the increasing ionic strength. This is because that the pore size increases when the ionic strength increases, resulting in the higher diffusion. The diffusion coefficient of Re(Ⅶ) is basically unchanged under different pH, this is because that ReO-4 is the main form under different pH. The irradiation will reduce the diffusion rate of Re(Ⅶ) in the clay, this may due to the influence of irradiation on the specific surface area, pore size and organic content of clay. The adsorption amount of Re(Ⅶ) in irradiated clay is larger than that in the Tamusu clay, which may be due to the increasing specific surface area and adsorption site of irradiated clay, and Re(Ⅶ) in the solution is more easily adsorbed on the irradiated clay.