In　this　study,　the　succession　and　distribution　of　biofilm　microbial　communities　in　different　functional　zones　of　an　A2O-IFAS　reactor　was　explored.　The　IFAS　reactor　was　used　to　treat　real　domestic　wastewater　and　the　total　inorganic　nitrogen　removal　loading　rate　reached　(0.10±0.03)　kg·m-3·d-1.　The　removal　efficiencies　of　total　inorganic　nitrogen　and　COD　were　70%　and　79%,　respectively.　The　high-throughput　sequencing　results　revealed　that　aerobic　biofilm　maintained　the　highest　relative　abundance　of　Candidatus_Brocadia,　followed　by　oxic　biofilm,　activated　sludge　and　anerobic　biofilm.　Further　analysis　suggested　that　the　microbial　community　of　the　activated　sludge　was　more　similar　to　aerobic　biofilm.　Besides,　non-biological　factors　in　functional　zones　drove　the　succession　of　microbial　community　in　anoxic　and　anaerobic　biofilm.　The　distribution　of　microbial　community　as　well　as　the　enrichment　of　anammox　bacteria　was　also　driven　and　determined　by　DO,　C/N　ratio　and　other　key　environmental　factors,　together　with　the　interaction　mechanisms　of　microorganisms.　This　study　provides　an　important　reference　for　the　enrichment　of　anammox　bacteria　and　technical　application　of　mainstream　anammox.　©　2022,　Science　Press.　All　right　reserved.