In　this　research,　the　numerical　model　of　fresh　self-compacting　concrete　(SCC)　U-box　test　was　established　by　combining　lattice　Boltzmann　method　(LBM)　and　Herschel-Bulkley　(H-B)　model.　The　flow　behavior　of　fresh　SCC　in　U-box　was　studied　and　its　rheological　mechanism　was　analyzed.　The　test　results　showed　that　the　simulation　model　established　based　on　U-box　test　had　high　simulation　accuracy,　and　the　average　error　was　about　7　%.　The　effects　of　rheological　parameters　(power-law　index　n,　consistency　index　k　and　yield　stress　sigma(0))　on　the　U-box　test　were　explored.　The　numerical　results　demonstrated　that　sigma　0　was　the　crucial　rheological　parameter　that　deter-mined　the　passing　ability　of　fresh　SCC,　and　its　increase　resulted　in　weaker　passing　ability.　n　and　k　had　less　effect　on　the　passing　ability.　At　the　early　stage　of　U-box　test　(t　＜　1.0　s),　the　increase　of　n,　k　and　sigma(0)　all　reduced　the　velocity　of　flow　and　the　pressure　exerted　on　the　fresh　SCC,　with　n　bringing　the　most　significant　effect.　The　peak　velocity　and　pressure　of　shear-thinning　(n　=　0.6)　SCC　could　reach　0.74　m/s　and　4200　Pa　at　t　=　0.1　s.　The　peak　velocity　of　shear-thickening　(n　=　1.4)　SCC　was　less　than　half　of　the　former,　and　a　larger　negative　pressure　appeared,　up　to　about-1600　Pa.　The　speed　of　pulling　up　the　partition　gate　(u(g))　only　had　an　effect,　which　gradually　diminished　with　the　increase　of　u(g),　on　the　initial　phase　of　U-box　test　(t　＜　5.0　s).　The　cross-sectional　area　corresponding　to　the　obstacles　that　could　be　passed　by　the　SCC　was　the　main　factor　affecting　the　early　velocity　and　pressure　of　the　concrete.