To　study　the　effect　of　slenderness　ratio　on　the　ultimate　bearing　capacity　of　UHPC　(ultra-high　performance　concrete)-encased　concrete-filled　steel　tubular　(UC-CFST)　columns,　a　total　of　nine　specimens　under　axial　compression　were　tested.　A　finite　element　model,　considering　the　effects　of　material　nonlinearity　and　interaction　between　concrete　and　steel　tube,　was　developed.　A　full-range　analysis　was　carried　out　on　the　typical　load-deformation　response　of　the　UC-CFST　medium-long　columns.　The　experimental　and　numerical　analysis　results　showed　that　with　the　increase　in　slenderness　ratio,　the　deformation　capacity　of　the　UC-CFST　columns　increased,　the　ultimate　bearing　capacity　decreased,　and　the　failure　mode　changed　from　exceedance　of　compressive　strength　to　loss　of　stability.　The　internal　forces　and　stress　distributions　at　the　mid-height　section　of　the　columns　at　different　stages　were　also　investigated.　Finally,　the　applicability　of　various　methods　for　calculating　the　ultimate　bearing　capacity　of　ordinary　concrete-encased　CFST　(OC-CFST)　columns　to　UC-CFST　columns　was　discussed,　including　the　methods　recommended　by　Chinese　standard　T/CECS　188-2019　(CABP,　2020),　European　standard　Eurocode　4　(CEN,　2004),　Australian/New　Zealand　joint　standard　AS/NZS　5100.6　:　2017　(JTC,　2017)　and　American　standard　ANSI/AISC　360-16　(　AISC,　2016).　It　was　found　that　the　method　recommended　in　AS/NZS　5100.6　:　2017　(JTC,　2017)　was　suitable　for　calculating　the　ultimate　bearing　capacity　of　UC-CFST　medium-long　columns.