To　investigate　the　confined　effect　and　axial　compression　strength　of　circular　steel　tube　confined　ultra　high　performance　concrete　(UHPC)　stub　columns,　axial　compression　tests　on　12　circular　steel　tube　confined　UHPC　stub　specimens　were　carried　out,　and　the　failure　mode,　deformation,　and　force　transfer　were　analyzed.　The　test　results　show　that　the　failure　mode　of　circular　steel　tube　confined　UHPC　stub　specimens　is　related　to　the　confinement　factor.　With　the　increase　of　confinement　factor,　the　specimens　show　shear　failure,　mixed　failure,　and　waist　drum　failure　in　order.　The　specimen　with　a　smaller　confinement　factor　(ranging　from　0.43　to　0.52)　has　a　shorter　elastic-plastic　stage　in　the　whole　failure　process,　showing　a　more　obvious　brittle　failure,　while　the　confinement　factor　of　the　specimen　is　large　(ranging　from　1.21　to　1.80),　the　plastic　damage　phenomenon　would　be　enhanced.　Based　on　the　finite　element　model　validated　by　experiments,　parametric　analysis　was　carried　out,　which　shows　that　the　ultimate　capacity　improvement　coefficient　varies　from　1.2　to　1.4.　In　the　range　of　0.43≤ξ≤1.08,　the　ultimate　capacity　improvement　coefficient　increases　with　the　increase　of　confinement　factor,　and　the　recommended　value　range　of　diameter-to-thickness　ratio　is　12.0-23.0.　By　comparing　with　the　existing　ultimate　capacity　formula,　the　predicting　formula　for　the　steel　tube　confined　UHPC　ultimate　capacity　was　modified　and　its　calculation　results　fit　well　with　the　experimental　results.　©　2022,　Editorial　Office　of　Journal　of　Building　Structures.　All　right　reserved.