The　interface　shear-resistant　construction　method　between　ultrahigh-performance　concrete　(UHPC)　and　normal　concrete　(NC)　is　critical　to　ensuring　the　overall　collaborative　work　performance　and　structural　safety　of　steel-UHPC-NC　composite　girders　(SUNCGs).　However,　the　influence　of　UHPC-NC　interface　construction　on　the　flexural　performance　of　SUNCGs　has　not　yet　been　explored.　In　this　research,　a　flexural　test　in　the　positive　moment　regions　was　first　carried　out　to　study　the　impact　of　different　UHPC-NC　interface　treatments　on　the　SUNCGs.　The　test　results　showed　that　SUNCGs　exhibited　excellent　flexural　performance　and　higher　flexural　capacity　than　the　steel-NC　composite　girder.　In　contrast　to　the　interface　shear　failure　observed　in　the　SUNCGs　with　3　and　6　mm　roughened　UHPC-NC　interfaces,　the　SUNCG　with　6　mm　roughness　and　adequate　shear　reinforcement　at　the　UHPC-NC　interface　fully　exerted　the　combined　action　of　its　composite　slab　and　ultimately　failed　in　plastic　bending.　Besides,　a　finite-element　analysis　was　also　performed　to　further　study　the　effects　of　interface　reinforcement　ratios　and　interface　cohesion　parameters.　Finally,　theoretical　formulas　for　evaluating　shear　characteristics　and　determining　the　minimum　reinforcement　ratio　at　the　UHPC-NC　interface　were　proposed.　The　test　results　validated　the　applicability　of　these　formulas　for　the　shear　design　of　the　UHPC-NC　interface　in　SUNCGs.　©　2025　American　Society　of　Civil　Engineers.