To　study　the　mechanical　performance　of　concrete-filled　square　steel　tube　stiffened　with　PBL　under　bending,　this　paper　describes　a　comparison　test　study　on　the　mechanical　behavior　of　three　types　of　concrete-filled　square　steel　tube　structure　under　bending.　The　first　type　is　concrete-filled　square　steel　tube,　the　second　type　is　concrete-filled　square　steel　tube　with　steel　plate　stiffening　rib,　and　the　third　type　is　concrete-filled　square　steel　tube　with　PBL　stiffening　rib.　The　influences　of　aperture　size　and　spacing　of　PBL　were　also　investigated.　Test　results　indicate　that　the　failure　mode　of　the　first　type　specimen　was　single　wave　buckling,　the　failure　modes　of　the　other　two　were　double　wave　buckling.　Concrete　crack　distribution　of　the　third　type　specimen　was　more　uniform,　and　the　width　and　spacing　were　smaller,　showing　steel　tube　and　concrete　worked　together　better.　The　bending　capacity　improved　by　PBL　stiffening　rib　was　not　obvious,　but　the　bending　rigidity　was　increased　by　about　20%　compared　with　the　first　type　specimen.　Whether　the　steel　plate　rib　has　pore　or　not　had　little　effect　on　the　rigidity　of　specimens.　FEA　results　demonstrate　that　the　bending　capacity　increased　with　the　increase　of　the　height　and　width　of　PBL　stiffening　rib,　but　decreased　with　the　increase　of　relative　pore　diameter　d0,　and　the　influence　of　pore　spacing　on　bearing　capacity　was　very　small.　Based　on　the　test　and　FEA　results,　a　simplified　calculation　method　of　concrete-filled　square　steel　tube　with　PBL　stiffening　rib　was　proposed,　and　the　value　of　relative　pore　diameter　and　relative　pore　spacing　were　recommended　as　0.5　and　2.0　respectively　in　practical　project.　©　2017,　Engineering　Mechanics　Press.　All　right　reserved.