Abstract: Compressive dense fault fracture zones have positive permeability and mechanical properties in their in-situ state, but 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, ignoring the regulatory effect of variations in filling medium characteristics on permeability behavior, which leads to deviations in leakage risk assessment. This study focuses on the F2 fault fracture zone on the right bank of the Yulong Kashu Hydroelectric Hub, employing an ultra-dense layered compaction method to prepare samples with stone content ranging from 15% to 60%, and systematically conducting permeability tests. We can conclude: (1) Stone content controls the structural characteristics of the filling medium. When the stone content is >40%, a rock skeleton-dominated structure is formed, and when <30%, a soil particle accumulation structure appears. The permeability failure modes of these two structures show significant differences. (2) As the Talbot index (n) increases, the critical hydraulic gradient decreases. However, under conditions where the stone content is ≥50%, the influence of n on the hydraulic gradient decrease weakens. (3) When the fine particle content exceeds 20%, the flocculation and gelation of viscous particles significantly improve the structural impermeability, with the average hydraulic gradient decrease in the experimental group increasing by 58% compared to the control group. 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.