This　study　investigates　the　flexural　behavior　of　the　large　prestressed　UHPC　box　girder.　An　overpass　carrying　a　section　of　the　Shijiazhuang-Ci　County　Expressway　in　Hebei　Province　is　used　as　a　case.　A　30　m-long　full-scale　prestressed　UHPC　box　girder　mock-up　was　prepared　for　the　four-point　flextural　tests,　together　with　the　finite　element　calculation,　the　findings　of　the　tests　as　well　as　multiple　key　factors　were　analyzed,　including　crack　distribution,　midspan　load-dispalcement　curve,　UHPC　in　tensile　zone,　and　the　stress-strain　curves　of　the　tendons.　In　the　flexural　tests,　the　UHPC　in　the　compressive　zone　was　not　crushed　when　the　box　girder　fractured,　the　steel　fibers　were　pulled　out　in　the　tensile　zone,　and　the　cracks　propagated　through　the　bottom　slabs　of　the　box　girder.　The　load-displacement　curves　indicate　that　the　box　girder　experienced　four　stages　during　the　test,　including　the　elastic　stage,　the　crack　propagation　stage,　the　crack　rapid　propagation　stage　and　failure　stage.　It　can　be　seen　from　the　stress-strain　curves　that　the　UHPC　in　the　tensile　zone　still　contributes　tensile　stress　even　when　the　longitudinal　tendons　were　yielded.　The　findings　in　the　flexural　tests　and　the　measured　mechanical　property　indexes　all　proved　that　there　was　large　flexural　performance　margin　of　the　prestressed　UHPC,　which　can　be　further　optimized.　The　optimization　of　the　prestressed　UHPC　box　girder　was　performed　considering　the　amount　of　common　reinforcing　bars,　prestressing　tenons　amount,　web　thickness　and　bottom　slab　thickness.　It　is　shown　that　the　UHPC　box　gider　with　no　normal　reinforcement　still　meets　the　design　requirements　of　ultimate　limit　state.　Increasing　the　number　of　prestressing　tendons　in　the　bottom　slabs　can　effectively　enhance　the　compressive　strength　utilization　rate　of　the　UHPC　in　compression　zones.　The　web　thickness　should　be　kept　within　proper　range,　thinner　webs　arc　more　likely　to　succumb　to　oversized　principal　tensile　stress,　thus　better　being　no　less　than　8　cm.　Adding　external　prestressing　tendons　arc　associated　with　the　reduction　of　the　thickness　of　bottom　slabs　of　the　box　gider.　©　2024　Wuhan　Bridge　Media　Co.,　Ltd.,　MBEC.　All　rights　reserved.