Biofilm　technology　is　the　key　to　the　application　of　anaerobic　ammonia　oxidation　process,　but　there　are　few　reports　on　the　characteristics　of　biofilms　formed　under　synergistic　effect　of　different　ammonia　nitrogen　concentrations　and　suspended　sludge.　In　this　study,　in　a　plug-flow　fixed　biofilm-activated　sludge　reactor,　the　biofilm　sludge　concentration　and　thickness　under　high　ammonia　nitrogen　concentration　conditions　were　greater　than　those　under　low　ammonia　nitrogen　concentration,　but　the　latter　had　higher　anammox　bacteria　abundance　((4.91±0.65)×109　copies·g−1,　P＜0.05)　and　activity　(6.53　mg·(g·h)−1).　High-throughput　analysis　showed　that　Candidatus　Brocadia　was　the　major　anaerobic　ammonia　oxidation　bacteria　in　both　biofilms　and　suspended　sludge,　and　there　was　no　significant　difference　in　abundance　between　the　two　types　of　biofilms.　The　relative　abundance　of　Candidatus　Jettenia　was　significantly　higher　in　biofilms　at　low　ammonia　nitrogen　concentrations　than　at　high　ammonia　nitrogen　concentrations,　but　the　abundance　of　Candidatus　Kuenenia　was　the　opposite.　Comprehensive　analysis　revealed　that　the　attachment　growth　of　anaerobic　ammonia　oxidation　bacteria　was　related　to　the　initial　colonization　of　suspended　sludge　community　diversity,　while　the　distribution　of　low　abundance　AnAOB　was　influenced　by　different　microhabitats.　This　study　suggests　that　the　synergistic　effect　of　different　ammonia　nitrogen　concentrations　and　suspended　sludge　types　on　biofilms　should　not　be　neglected.　©　2024　Science　Press.　All　rights　reserved.