Taking　the　steel　content　and　hoop　coefficient　as　test　parameters,　the　pure　bending　tests　of　5　circular　ultra-high　performance　concrete　(UHPC)　filled　high　strength　steel　tube　(UFHST)　beams　and　2　circular　UHPC　filled　normal　strength　steel　tube　(UFST)　beams　were　carried　out　to　investigate　their　bending　behaviors.　The　experimental　results　show　that　UFHST　beams　failed　in　ductile.　The　composite　section　satisfied　the　assumption　of　plane　cross-section.　The　hoop　action　of　the　high　strength　steel　tube　on　UHPC　should　be　considered　in　the　compression　zone.　Compared　with　the　normal　strength　steel　tube,　high　strength　steel　tube　can　restrain　the　transverse　expansion　of　UHPC　more　effectively.　The　finite　element　model　verified　by　test　results　was　used　to　conduct　parameter　analysis　of　the　hoop　coefficient.　When　the　concrete-filled　steel　tube　beam　reaches　its　bending　bearing　capacity,　the　corresponding　neutral　axis　and　the　stresses　of　steel　and　concrete　are　related　to　the　hoop　coefficient.　With　the　increase　of　the　hoop　coefficient,　the　neutral　axis　keeps　moving　toward　the　section　centerline;　there　is　a　decrease　in　the　steel　stress　in　the　tensile　zone,　and　an　increase　in　the　steel　stress　and　concrete　stress　in　the　compressive　zone.　It　was　inaccurate　and　discrete　to　use　Chinese　code　GB50936−2014　for　predicting　the　bending　bearing　capacity　of　UFHST　beams.　Consequently,　a　new　calculation　method　was　proposed.　Copyright　©2021　Engineering　Mechanics.　All　rights　reserved.