After　using　Ultra-High　Performance　Concrete　(UHPC),　the　mechanical　properties　of　the　composite　beams　have　significant　differences　compared　to　those　of　the　steel-normal　concrete　composite　beams.　To　investigate　the　effect　of　reinforcement　ratio　on　the　force　behavior　of　steel-UHPC　composite　beams,　tests　on　the　flexural　performance　of　steel-UHPC　composite　beams　with　different　reinforcement　ratios　under　negative　moments　were　carried　out,　and　reasonable　design　of　the　composite　beams　was　analyzed　by　using　a　numerical　simulation　method.　In　addition,　the　existing　crack　calculation　methods　for　composite　beams　were　validated.　The　results　showed　that　the　failure　modes　of　the　composite　beams　with　different　reinforcement　ratios　were　similar.　Increasing　the　reinforcement　ratio　from　2.01%　to　2.98%,　the　ultimate　capacity　of　the　steel-UHPC　composite　beams　was　increased　by　4%　and　the　ultimate　deflection　was　reduced　by　7%,　while　the　ductility　was　reduced　by　11%.　The　cracking　load　was　not　enhanced,　but　the　higher　reinforcement　ratio　had　a　controlling　effect　on　crack　development　and　crack　width　in　the　late　loading　stage.　For　steel-UHPC　composite　beams,　the　appropriate　reinforcement　ratio　should　be　less　than　4%.　Furthermore,　the　crack　calculation　formulas　in　the　existing　steel-concrete　composite　beam　codes　do　not　apply　to　steel-UHPC　composite　beams,　and　the　calculation　results　were　much　smaller　than　the　measured　values.　Among　the　related　formulas　for　reinforced　UHPC　members,　the　calculated　value　of　the　formula　in　NF　P　18-710　was　closer　to　the　measured　value　but　the　overall　error　was　still　large,　so　further　research　is　needed.