To　study　the　eccentric　compression　performance　and　failure　mechanism　of　ultra-high　performance　concrete　(UHPC)　encased　concrete-filled　steel　tubular　(CFST)　medium-long　columns　(UC-CFST　medium-long　columns),　a　total　of　27　UC-CFST　column　specimens　were　tested　under　eccentric　compression　with　eccentricity　and　slenderness　ratio　as　the　main　parameters.　After　verified　by　experimental　results,　the　finite　element　(FE)　analysis　of　UC-CFST　medium-long　columns　with　actual　structural　cross-sectional　size　under　eccentric　compression　was　further　carried　out.　It　is　found　that　as　the　slenderness　ratio　and　eccentricity　increased,　the　bending　stiffness　and　the　ultimate　bearing　capacity　of　the　UC-CFST　columns　decreased　gradually,　and　the　failure　mode　changed　from　strength　failure　to　instability　failure,　and　the　smaller　the　proportion　of　encasing　UHPC　area,　the　greater　the　upper-bound　slenderness　ratio　of　strength　failure.　Moreover,　the　influence　of　eccentricity　was　nonlinearly　coupled　with　slenderness　ratio　on　the　eccentric　compressive　bearing　capacity　of　UC-CFST　medium-long　columns.　Finally,　a　calculation　method　of　ultimate　load-bearing　capacity　of　UC-CFST　medium-long　columns　considering　the　influence　of　eccentricity　and　slenderness　ratio　was　proposed,　which　had　good　calculation　accuracy　with　experimental　and　numerical　results.