A　novel　double-anammox　process　for　advanced　mainstream　nitrogen　removal　was　established　using　step-feed　sequencing　batch　reactor　(SBR)　system　with　integration　of　suspend　sludge　and　biofilms.　Following　optimization　of　influent　distribution　ratio,　the　effluent　total　inorganic　nitrogen　(TIN)　was　＜　10.2　mg　N/L,　with　influent　TIN　of　43.4　mg　N/L,　and　anammox　contributed　71.4%　to　TIN　removal.　Biological　processes　and　batch　tests　revealed　that　gradient　C/N　reduction　promoted　denitratation/anammox　in　anoxic　stage,　and　simultaneous　nitritation　and　anammox　were　achieved　in　oxic　stage.　Specially,　anammox　maintained　on　biofilms　with　abundance　over　109　copies/　(g　dry　sludge).　High-throughput　sequencing　revealed　that　Thauera　and　Nitrosomonas　were　enriched　in　flocs.　Furthermore,　metagenomic　sequencing　confirmed　that　Thauera　owns　narG　and　napA　(NO3--＞　NO2-)　and　Nitrosomonas　owns　amoA　(NH4+-＞　NO2-),　support　stable　NO2-　supply　for　double-anammox.　This　mainstream　anammox-dominant　process　could　potentially　be　used　for　stable　nitrogen　removal　in　municipal　wastewater　treatment　plants.