The　basic　premise　of　anammox-technical　application　reliability　in　municipal　wastewater　treatment　is　substantially　enriched　anammox　bacteria.　To　enrich　the　anammox,　the　special　interaction　mechanism　between　the　suspended　sludge　(SS)　and　anoxic　biofilm　was　investigated　over　three　months　in　a　partial　denitrification/anammox　biosystem　subjected　to　dynamic　changes　in　SS　(absence　-＞　presence　-＞　absence).　Results　show　that　the　introduction　of　SS　significantly　decreased　the　anammox　nitrogen　removal　efficiency　(83.8　+/-　6.5%-＞　48.7　+/-　17.0%).　With　the　presence　or　absence　of　SS,　the　spatial　distribution　of　anammox　bacteria　within　the　anoxic　biofilm　gradually　changed　between　the　inner　and　outer　layers,　as　detected　by　CLSM-FISH.　qPCR　and　meta-genomic　sequencing　show　that　changes　in　the　presence　and　absence　status　of　SS　significantly　reduced　the　abundance　of　the　NO　reducing　functional　gene,　while　the　NO　supply　capacity　(NO3--＞　NO)　was　improved,　further　favoring　the　anammox　process.　Batch　tests　and　typical　cycles　further　demonstrated　that　the　anammox　bacteria　can　stably　acquire　NO2-,　and　anammox　bacteria　in　the　anoxic　biofilm　competed　far　more　NO2-　than　denitrifying　bacteria　according　to　the　typical　pH　curve.　Accordingly,　the　abundance　of　Candidatus　Brocadia,　as　detected　by　high　throughput　sequencing,　decreased　in　the　anoxic　biofilms　with　the　introduction　of　SS,　but　greatly　increased　(0.82%-＞　2.22%)　after　SS　discharge.　This　study　sheds　new　light　on　the　high　in-situ　enrichment　of　anammox　in　mainstream.