Steel　fiber　can　reinforce　the　compressive　strength　of　ultra-high　performance　fiber　reinforced　concrete　(UHPFRC),　but　it　will　reduce　the　flowability　of　fresh　UHPFRC,　thus　weaken　this　reinforcement　effect　and　the　work　performance　of　UHPFRC.　In　order　to　study　this　adverse　effect,　two　sets　of　tests　A　and　B　were　carried　out　with　steel　fiber　volume　fraction　and　aspect　ratio　as　variables.　In　the　A　series　test,　the　fixed　water-to-binder　ratio　was　0.18　and　didn＇t　control　the　flowability.　It　mainly　studied　the　effect　of　steel　fiber　on　flowability　and　compressive　strength.　The　test　results　show　that　the　flowability　of　fresh　UHPFRC　decreases　with　the　increase　of　the　volume　fraction　or　aspect　ratio　of　steel　fiber;　when　the　volume　fraction　of　steel　fiber　exceeds　a　certain　value　(2.00vol%),　the　expansion　degree　decreases　significantly,　and　the　corresponding　compressive　strength　enhancement　effect　also　decreases.　The　X-ray　CT　scan　test　finds　that　the　incorporation　of　steel　fiber　weaken　the　self-compacting　ability　of　the　fresh　slurry,　increasing　in　the　internal　pore　size　and　porosity　of　the　hardened　matrix,　thereby　weakening　the　compressive　strength.　Comprehensively　considering　the　positive　and　negative　effects　of　steel　fiber　incorporation　into　compressive　strength,　a　semi-empirical　prediction　formula　for　compressive　strength　was　proposed.　In　the　B　series　test,　the　water-binder　ratio　was　changed　and　the　expansion　degree　was　controlled　to　240　mm.　Through　comparative　analysis　of　the　A　series　test　results,　the　effect　of　the　volume　fraction　and　aspect　ratio　of　the　steel　fiber　on　the　compressive　strength　after　the　control　of　the　flowability　was　studied.　The　results　show　that　when　the　volume　fraction　of　steel　fiber　is　large,　the　water-binder　ratio　should　be　increased　to　ensure　a　certain　flowability,　which　can　effectively　improve　the　reinforcement　effect　of　the　fiber;　when　the　volume　fraction　of　fiber　is　small,　the　water-binder　ratio　should　be　decreased　on　the　premise　of　meeting　the　flowability　requirements,　which　can　further　improve　the　compressive　strength.　In　the　design　of　the　UHPFRC　mix　ratio,　in　order　to　improve　the　reinforcing　effect　of　steel　fiber　and　ensure　that　UHPFRC　has　good　working　performance,　the　adverse　effect　of　incorporating　steel　fiber　on　the　flowability　of　UHPFRC　should　be　considered.　©　2021,　Editorial　Office　of　Acta　Materiae　Compositae　Sinica.　All　right　reserved.