Objective　The　granite　weathering　process,　influenced　by　internal　and　external　forces　from　various　geological　effects,　the　distribution　of　irregular　joints,　and　other　factors,　causes　the　granite　weathering　shell　to　exhibit　differential　weathering.　This　results　in　the　formation　of　a　granite　binary　structure　of　slopes,　with　the　upper　part　of　the　slope　consisting　of　weathered　soil　and　the　lower　part　of　weathered　rock　and　discontinuous　differences　between　the　upper　and　lower　parts　at　the　weathering　interface.　Weathered　soil　is　the　main　source　of　granite　binary-structured　slopes,　and　clarification　of　its　internal　erosion　characteristics　is　the　basis　for　studying　the　damage　mode　and　slip　promotion　mechanism　of　this　type　of　slope.　The　internal　erosion　of　granite-weathered　soil　under　groundwater　seepage　involves　fine　particle　migration　and　siltation.　Due　to　the　non-homogeneity　of　granite-weathered　soil　and　the　randomness　of　particle　migration,　existing　studies　cannot　fully　reveal　the　internal　erosion　process　and　particle　migration　law　of　granite-weathered　soil.　Additionally,　there　are　fewer　studies　on　the　interaction　process　between　fine　particle　transport　and　pore　plugging　inside　the　soil　under　dynamic　head　conditions,　which　limits　further　understanding　of　particle　migration　and　internal　erosion　processes　in　granite-weathered　soil.　Methods　This　study　focuses　on　the　granite　slope　next　to　County　Road　X135　in　Huangqi　Town,　Lianjiang　County,　Fujian　Province.　Understanding　the　geological　phenomena　of　internal　erosion　in　weathered　soil　clarifies　the　environment　and　triggering　factors　of　internal　erosion　in　granite-weathered　soil.　A　lateral　seepage　test　is　conducted　using　a　sandbox,　combined　with　discrete　element　numerical　simulations　to　reproduce　the　evolution　of　the　internal　erosion　process.　This　reveals　the　dynamic　processes　of　fine　particle　migration　and　siltation,　investigates　the　influence　of　hydraulic　conditions　on　internal　erosion　development,　and　summarizes　the　mechanisms　of　internal　erosion　in　granite-weathered　soil.　Results　and　Discussions　The　results　show　that　soil　erosion　is　a　gradual　development　process.　In　the　initial　stage　of　seepage,　the　bonding　of　fine　particles　at　weak　points　in　the　soil　is　weak,　making　them　highly　susceptible　to　loss　under　water　flow.　Over　time,　the　soil　gradually　stabilizes,　and　water　flow　preferentially　follows　established　seepage　channels.　From　a　macroscopic　perspective,　more　active　water　movement　results　in　more　developed　seepage　pore　channels.　At　a　finer　scale,　fine　particle　migration　during　erosion　is　irregular,　expanding　the　pore　structure　while　also　causing　blockages　or　deposition　at　the　bedrock　interface.　Under　dynamic　head　actions,　fine　particles　continue　to　migrate,　forming　pore　cavities　within　the　soil,　which　eventually　connect　to　form　seepage　channels,　accelerating　water　transport.　Increasing　the　amplitude　of　head　change　and　the　seepage　inclination　angle　induces　greater　soil　particle　migration　and　loss,　exacerbating　internal　erosion　and　prolonging　the　stabilization　time　of　the　soil.　The　composition　and　proportion　of　lost　particles　in　weak　zones　are　very　similar　to　those　of　the　upper　weathered　soil,　indicating　that　the　weak　zones　form　through　fine　particle　migration　and　deposition　under　seepage　effects.　This　self-filtering　process　reflects　the　dynamic　interaction　of　particle　migration　and　deposition　in　granite　weathered　soil.　Conclusions　Under　long-term　rainfall,　the　fully　weathered,　fine-grained　clay　particles　in　granite　weathered　soil　aggregate　at　different　weathering　interfaces.　The　long-term　softening　effect　of　groundwater　forms　weak　structural　surfaces　that　control　slope　stability.　These　weak　structural　surfaces　divide　the　slope　into　an　upper　layer　of　granite　weathered　soil　and　a　lower　layer　of　strongly　weathered　granite,　forming　a　binary　structure.　During　the　self-filtering　process,　the　coarsening　of　soil　particles　caused　by　fine　particle　migration　easily　triggers　soil　collapse　and　damage.　Additionally,　the　softening　of　the　base-cover　interface　due　to　fine　particle　deposition　evolves　into　a　critical　factor　controlling　slope　stability.　These　factors　are　the　primary　causes　of　instability　in　granite　binary-structured　slopes.　This　study　preliminarily　clarifies　the　mechanisms　of　internal　erosion　in　granite　weathered　soil　and　provides　a　theoretical　basis　for　predicting　slope　instability　and　implementing　disaster　prevention　and　mitigation　measures　for　granite　slopes.　©　2025　Sichuan　University.　All　rights　reserved.