Based　on　the　ultra-high　filling　project　of　Yuxi-Mohan　Railway　Pu＇er　Station,　the　ABAQUS　finite　element　software　was　used　to　study　the　loading　mode　and　subgrade　stability　of　traditional　rigid　pile　and　flexible　pile　composite　foundation　under　ultra-high　fill　embankment　load.　The　PHC　pipe　pile　in　the　center　of　the　traditional　rigid　pile　composite　foundation　embankment　is　low　in　shear　strength　and　bending　moment,　and　the　bending　and　shear　properties　of　PHC　pipe　piles　are　not　fully　utilized;　The　traditional　flexible　pile　composite　foundation　is　characterized　by　large　lateral　deformation　value　of　the　embankment　side　pile,　flexible　bending　of　the　pile　body　and　relatively　low　safety　of　the　embankment.　In　order　to　ensure　the　safety　of　embankment,　and　at　the　same　time　take　into　account　the　engineering　cost,　an　optimal　design　method　for　composite　foundation　of　rigid-flexible　composite　pile　under　ultrahigh　fill　load　was　proposed.　The　rigid　pile　was　arranged　at　the　location　of　the　potential　slip　surface　shearing　or　shearing　exit,　and　the　flexible　pile　at　the　rest　of　the　position　was　arranged.　Then　its　reinforcement　mechanism　and　effect　were　analyzed　by　finite　element　numerical　simulation　method.　The　results　show　that　the　rigid　pile　is　arranged　at　the　shearing　entrance　position,　which　can　significantly　reduces　the　lateral　deformation　of　the　embankment　and　the　vertical　deformation　of　the　top　surface　of　the　embankment　at　the　shoulder,　and　its　existence　can　directly　prevent　further　development　and　penetration　of　potential　slip　surfaces.　And　the　rigid　pile　is　arranged　at　the　shearing　exit　position,　which　can　reduce　the　lateral　deformation　of　the　embankment,　but　the　effect　is　not　significant,　and　the　existence　can　share　the　shear　stress　of　the　soil　around　the　pile　at　the　shearing　outlet,　so　that　the　plastic　zone　is　dispersed　around.　And　the　safety　factor　values　of　the　embankments　under　the　two　optimization　schemes　are　significantly　improved　compared　with　the　conventional　flexible　pile　composite　foundation.　©　2019,　Central　South　University　Press.　All　right　reserved.