The　steel-ultra-high　performance　concrete　(UHPC)　composite　girder,　which　is　composed　of　UHPC　slab　and　steel　girder,　has　the　advantages　of　light　self-weight,　high　cracking　resistance　and　durability,　and　it　is　of　great　significance　for　the　development　of　steel-concrete　composite　girders.　To　investigate　the　difference　of　mechanical　performance　of　a　steel-UHPC　composite　girder　and　steel-conventional　concrete　under　bending,　in　this　paper,　a　trial-design　of　steel-UHPC　composite　girder　bridge　was　carried　out　based　on　the　prototype　structure　of　a　practical　steel-concrete　simply　supported　composite　girder　bridge,　according　to　the　trial-design　results,　the　experimental　study　on　flexural　behaviors　of　steel-UHPC　composite　girder　and　steel-conventional　concrete　composite　girder　were　carried　out.　Test　results　show　that　flexural　behaviors　of　two　kinds　of　composite　girder　were　similar.　Both　of　their　failure　modes　were　that　the　bottom　flange　of　steel　girder　was　yielded　first,　and　then　the　upper　concrete　of　slab　was　crushed.　Compared　with　a　steel-conventional　concrete　composite　girder,　under　the　condition　of　equal　ultimate　flexural　capacity,　the　slab　thickness　of　steel-UHPC　composite　girder　can　be　reduced　by　28%,　and　the　ductility　is　better.　The　shear　lag　effect　and　the　relative　slip　between　steel　girder　and　concrete　slab　of　steel-UHPC　composite　girder　are　much　smaller　than　those　of　a　steel-conventional　concrete　composite　girder.　Moreover,　the　flexural　stiffness　of　the　two　kinds　of　composite　girder　are　close　in　an　elastic　range.　However,　due　to　the　decrease　of　total　depth　of　a　steel-UHPC　composite　girder,　its　flexural　stiffness　is　some　smaller　in　an　elastic-plastic　range.　The　research　findings　of　this　paper　can　provide　a　reference　for　further　study　and　engineering　practice.　©　2018,　Engineering　Mechanics　Press.　All　right　reserved.