Permeability is one of the critical parameters for evaluating the hydraulic properties of water-conducting media. Variations in the hydraulic properties generally lead to uncertainties in the groundwater flow and solute transport in regional rock masses, influencing the migration and diffusion of high-level radioactive waste in hard-rock repositories. This study mainly focuses on the hydraulic properties of the Shiyuejing fault (about 25 km long), which is located in the Jiujing Block, i.e., one of the main candidate sites for a high-level radioactive waste repository in China. Fluid flow tests were conducted on large-scale irregular specimens (~0.008 m³) extracted from outcrops in the Shiyuejing fault zone via a nonstandard apparatus to determine the variations in the hydraulic properties. The dominant flow pathways were identified by slicing the specimens and extracting the geometric features of water-conducting fractures inside the specimens using a staining solution. The results show that three fracture sets exist in the fault zone, where the hydraulic conductivities exhibit a downward trend, from >10⁻⁴ to 10⁻⁵–10⁻⁴ cm/s to below 10⁻⁵ cm/s when sets I, II, and III are dominant, respectively. Three types of flow states, namely, laminar flow type, filling type, and erosion type, are indicated by the corresponding characteristics of the pressure-flow rate curves, and the permeabilities of these three flow types successively decrease.