Abstract: Compressive dense fault fracture zones exhibit good permeability and mechanical properties in their in-situ state. However, they are prone to forming leakage channels under the influence of high hydraulic gradients in reservoir areas. Current laboratory experiments often simplify such fault fracture zones into high-permeability gravel models, overlooking the regulatory effect of variations in filling medium characteristics on permeability behavior, which results in deviations in leakage risk assessment. This study focused on the F2 fault fracture zone on the right bank of the Yulong Kashi Hydroelectric Hub. An ultra-dense layered compaction method was employed to prepare samples with stone content ranging from 15% to 60%, and a series of systematic permeability tests were conducted. The key findings are as follows: ① Stone content controls the structural characteristics of the filling medium. When the stone content exceeds 40%, a rock skeleton-dominated structure is formed; when it is below 30%, a soil particle accumulation structure appears. These two structures exhibit significantly different permeability failure modes.②As the Talbot index (n) increases, the critical hydraulic gradient decreases. However, under conditions where the stone content exceeds 50%, the influence of n on the hydraulic gradient is weakened.③ When the fine particle content exceeds 20%, the flocculation and gelation of fine particles significantly improve the structural impermeability. The research results provide an important theoretical foundation and data support for optimizing the anti-seepage structure on the right bank of the dam site.