To　study　the　eccentric　compression　performance　of　UHPC-encased　concrete-filled　steel　tube　composite　medium-long　columns　(referred　to　as　‘UC-CFST　medium-long　columns’),　the　eccentricity　(0.　52　and　1.　04)　and　slenderness　ratio　(7.　22-44.　74)　were　selected　as　the　main　parameters,　and　a　total　of　18　UC-CFST　medium-long　column　specimens　were　tested　under　eccentric　compression　loads,　and　the　failure　mode　and　eccentric　compression　performance　index　were　obtained.　The　results　reveal　that　with　the　increase　of　eccentricity　and　slenderness　ratio,　the　failure　mode　of　UC-CFST　medium-long　columns　changes　from　material　strength　failure　to　instability　failure,　and　the　bearing　capacity　of　specimens　is　significantly　reduced.　The　specimens　with　a　slenderness　ratio　less　than　or　equal　to　21.　03　exhibit　obvious　material　strength　failure　characteristics.　The　steel　bars　and　steel　tubes　on　the　near　load　side　of　the　small　eccentric　compression　specimens　first　undergo　compression　yielding,　while　the　UHPC　outer　layer　in　the　tensile　zone　of　the　large　eccentric　compression　specimens　cracks,　and　the　steel　tubes　yield　in　tension.　When　the　peak　load　is　reached,　the　UHPC　in　the　compressive　zone　of　the　large　and　small　eccentric　compression　specimens　is　crushed.　The　specimens　with　a　slenderness　ratio　greater　than　35.　26　are　subjected　to　elastic-plastic　instability　failure,　while　a　specimen　with　a　slenderness　ratio　between　21.　03　and　35.　26　is　subjected　to　plastic　instability　failure.　Finally,　the　applicability　of　the　bending　moment　amplification　factor　method　for　concrete-encased　CFST　columns　proposed　in　GB/　T　51446—2021‘Technical　standard　for　concrete-filled　steel　tubular　hybrid　structures’　to　calculate　the　eccentric　compression　bearing　capacity　of　UC-CFST　medium-long　columns　is　discussed.　It　is　found　that　this　method　will　significantly　overestimate　the　eccentric　compression　bearing　capacity　of　UC-CFST　medium-long　columns.　Therefore,　combined　with　the　mechanical　characteristics　of　UC-CFST　medium-long　columns,　an　improved　moment　amplification　factor　method　which　can　be　used　to　calculate　the　eccentric　compression　bearing　capacity　of　UC-CFST　medium-long　columns　is　derived　in　this　paper.　The　calculation　results　correspond　well　with　the　experimental　results.　©　2024　Science　Press.　All　rights　reserved.