Abstract: The present study aims to systematically evaluate the barrier performance of the Xinchang granite and its fracture system in Beishan candidate area of China’s high-level radioactive waste(HLRW) deep geological disposal repository against key radionuclides. By adopting the internationally recognized constant-source through-diffusion experimental method, migration parameters of HTO(as a reference tracer) and two key radionuclides, ²³⁸Pu and ⁹⁹Tc, were determined in both media under strictly controlled low-oxygen experimental conditions(O₂ concentration＜1×10⁻⁶). The experimental results show that effective diffusion coefficients present obvious variability in the intact Xinchang granite samples: D_e(HTO)=(2.1±0.0)×10⁻¹³ m²/s＞D_e(Tc)=(5.3±2.1)×10⁻¹⁴ m²/s＞D_e(Pu)=(2.5±0.5)×10⁻¹⁴ m²/s. This suggests that, compared to HTO, a neutral molecule, \mathrmTcO_4^- anions and Pu(Ⅳ) colloids/ions are more strongly retarded. Notably, the diffusion coefficients increase significantly in the fracture infill, as demonstrated by: D_e(HTO)=(3.9±0.3)×10⁻¹⁰ m²/s＞D_e(Tc)=(2.9±0.3)×10⁻¹⁰ m²/s＞D_e(Pu)=(1.1±0.4)×10⁻¹² m²/s. Comparative analysis reveals that the diffusion coefficients in granite are on average 2-3 orders of magnitude lower than those in fracture infill. This mainly results from the much higher porosity in fracture infill, as it is not compacted in the diffusion column. This, in turn, directly confirms the critical controlling role of rock mass integrity on nuclide migration. Compared with HTO, both intact granite and fracture filling materials show some retardation effect on Pu and Tc migration, especially Pu, which may indicate that more complex reaction exist to immobilize Pu. It is particularly worrying that under the low oxygen conditions, ²³⁸Pu mainly exists in the form of Pu(OH)₄ colloids, and its effective diffusion coefficient decreases by about 2 orders magnitude compared to HTO, indicating its extremely strong retention in the granite system. Although ⁹⁹Tc migrated as \mathrmTcO_4^- anions, part of ⁹⁹Tc(Ⅶ) may be reduced to less soluble ⁹⁹Tc(Ⅳ) by the reductive effect of iron minerals in Beishan granite. This study confirms through systematic diffusion experiments that the Xinchang New Field granite system has significant natural retardation for key radionuclides especially the intact granite section, which constitutes an effective barrier for nuclide migration. These experimental data provide important baseline parameters for the safety assessment of China’s HLRW disposal repository and also provide a scientific basis for the optimization of engineering barrier design for the repository. It is recommended that the migration behavior of radionuclides under the coupling action of temperature-stress-chemical field should be further considered in subsequent research to more comprehensively evaluate the long-term safety of the disposal system.