In　this　paper,　12　ultra-high　performance　concrete　(UHPC)　filled　high-strength　steel　tubular　(UHPCFST)　short　columns　with　square-section　were　subjected　to　axial　compression　tests.　The　effects　of　confinement　factor　(xi)　on　the　axial　compression　performance　of　UHPCFST　were　discussed,　and　the　failure　modes,　axial　load-strain　curves,　and　strain　responses　were　discussed.　The　results　showed　that　the　specimens　mainly　had　two　failure　modes:　shear　failure　and　waist　drum　failure,　and　all　specimens　showed　good　ductility　and　high　post-peak　bearing　capacity,　indicating　that　high-strength　steel　tubes　could　provide　higher　restraint　for　UHPC　and　form　a　high-strength　and　high-performance　steel-UHPC　system.　When　xi　＞=　1.42,　the　strength　index　(SI)　increased　significantly　with　the　increase　of　xi.　When　xi　＜=　1.06,　the　SI　was　close　to　1.　With　the　increase　of　xi,　the　ductility　and　toughness　of　the　specimens　were　improved.　Finally,　considering　the　non-uniformity　of　the　confinement　of　the　core　UHPC　by　the　square-section　steel　tube,　a　calculation　model　of　the　ultimate　compressive　capacity　of　UHPCFST　specimens　with　square-section　was　proposed.　Compared　with　the　seven　classical　models,　the　calculation　model　proposed　in　this　paper　was　more　accurate.