Weathered　soil,　as　the　primary　source　material　in　granite-like　binary-structure　slopes,　plays　a　critical　role　in　slope　stability.　Understanding　its　internal　erosion　characteristics　is　essential　for　elucidating　the　failure　modes　and　sliding　mechanisms　of　granite　slopes.　Under　groundwater　seepage,　internal　erosion　in　weathered　granite　soil　involves　processes　such　as　fine　particle　migration　and　clogging.　In　this　study,　model　tests　and　numerical　simulations　were　conducted　to　investigate　particle　migration　by　examining　particle　velocity,　displacement,　loss,　and　changes　in　porosity　within　the　seepage　zone.　The　results　reveal　the　evolutionary　process　of　internal　erosion　in　weathered　granite　soil　and　the　multi-scale　transport　behaviour　of　fine　particles.　Under　seepage,　fine　particles　migrated　through　pore　spaces　between　the　particle　skeletons.　When　obstructed　by　the　skeletal　framework　or　bedrock　surface,　particle　movement　slowed,　leading　to　siltation　and　clogging.　During　seepage,　the　rate　of　particle　loss　decreased　progressively　over　time.　The　greatest　loss　occurred　within　the　first　hour,　with　over　90%　of　the　migrating　particles　consisting　of　clay　and　fine　sand　(0.25 mm　≤　d).　Particle　displacement　originated　in　structurally　weaker　regions　of　the　soil,　and　the　erosion　extent　expanded　as　seepage　continued.　The　migration　of　soil　particles　altered　the　internal　structure,　affecting　both　porosity　and　permeability.　This　study　provides　new　insights　into　the　mechanisms　of　internal　erosion　in　weathered　granite　soil,　providing　a　theoretical　basis　for　predicting　slope　instability　and　mitigating　geohazards.　©　The　Author(s),　under　exclusive　licence　to　Springer　Nature　Switzerland　AG　2025.