Due　to　its　structural　efficiency,　durability,　and　cost-effectiveness,　ultra-high　performance　concrete　was　utilized　to　build　the　first　highway　overpass　bridge　in　China.　The　bridge　was　made　of　prestressed　ultra-high　performance　concrete　box　girders　of　four　continuous　spans　of　30　m　each.　As　the　original　design　of　such　bridge　was　observed　to　be　somewhat　conservative,　its　cross-sectional　dimensions,　in　the　form　of　the　box　girder　wall　thicknesses　were　optimized　in　this　research　to　lower　the　material　cost　in　future　bridge　construction.　Then,　a　full-scale　simply　supported　ultra-high　performance　concrete　box　girder　of　30　m　span,　incorporating　the　new　box　girder　wall　thicknesses,　was　fabricated　and　then　tested　under　static　loading　to　obtain　research　data　to　justify　the　revised　design.　The　loading　system　was　designed　to　examine　the　flexural　behavior　of　the　girder　using　two　concentrated　loads　symmetrically　located　at　the　mid-span.　Experimental　results　show　that　the　optimized　girder　has　a　favorable　ductile　behavior　and　excellent　flexural　strength,　which　can　meet　the　design　requirements　for　serviceability　and　ultimate　limit　states.　A　finite　element　model　of　the　tested　girder　was　developed,　using　ABAQUS　software,　and　then　was　verified　using　the　experimental　findings.　A　parametric　study　was　then　conducted　to　investigate　the　influence　of　key　parameters　on　the　structural　response,　namely,　the　reinforcement　ratio,　the　number　of　the　prestressing　wires,　and　the　web　thickness.　Recommendations　on　minimum　and　maximum　compressive　strength　and　tensile　property　of　ultra-high　performance　concrete　were　proposed.　Also,　a　simplified　calculation　method　of　prestressed　ultra-high　performance　concrete　box　girder　was　developed　based　on　a　verified　strain　and　stress　diagrams　for　cross-sectional　analysis.　The　proposed　methodology　can　be　used　in　future　practice　with　confidence.　©　The　Author(s)　2019.