The　convex　steel　box　section　of　arch　rib　for　Yongjiang　Bridge　in　East　Outer　Ring　Road　of　Ningbo　City　was　taken　as　object,　the　1/4　scale-model　loading　experiment　of　arch　rib　segment　was　conducted　by　using　the　self-balanced　loading　mode　of　prestressed　steel　strand.　A　finite　element　model　was　built　based　on　shell　and　solid　elements,　and　nonlinear　analysis　was　carried　out.　The　mechanical　characteristic,　actual　bearing　capacity　and　local　instability　mechanism　of　convex　steel　box　section　of　arch　rib　were　studied　considering　initial　imperfections　and　local　buckling.　Analysis　result　shows　that　the　measured　stress　of　each　measurement　point　is　close　to　section　average　stress,　and　measurement　points　can　be　categorized　into　four　kinds　according　to　the　relationship　between　measured　stress　and　section　average　stress.　The　stresses　from　experiment　and　finite　element　analysis　show　the　same　tendency　and　similar　value.　The　strength　reduction　coefficient　of　convex　steel　box　section　of　arch　rib　is　0.94-0.98.　The　local　buckling　of　stiffened　plate　can　be　prevented　effectively　by　longitudinal　stiffeners　and　diaphragms.　Segment　has　wave　shape　under　ultimate　load,　and　fails　to　resist　applied　load　due　to　the　larger　out-of-plate　deformation　of　each　stiffened　plate.