The　flexural　behavior　of　12　ultra-high-performance　concrete　(UHPC)　encased　concrete-filled　steel　tubular　(CFST)　beams　was　experimentally　investigated　with　steel　fiber　content.　The　influence　of　steel　fiber　content　and　CFST　diameter　on　the　general　structural　behavior,　failure　modes,　stiffness,　and　flexural　capacity　were　discussed.　The　test　results　showed　that　the　outer　UHPC　can　work　in　good　coordination　with　the　inner　CFST,　and　steel　fiber　is　effective　in　improving　the　overall　flexural　performance　of　UHPC　encased　CFST　beams.　The　predictions　of　flexural　stiffness　by　existing　code　provisions　were　compared　with　the　test　results.　In　addition,　numerical　analysis　was　conducted　to　benchmark　test　results,　and　to　perform　an　in-depth　analysis　of　the　stress　development　and　load　distribution　between　UHPC,　steel　and　core　concrete.　The　applicability　of　existing　equilibrium　section　method　in　predicting　the　flexural　strength　of　UHPC　encased　CFST　beams　was　then　evaluated,　and　a　reduction　factor　was　proposed　to　calibrate　the　calculation　results.