At　normal　temperature　of　(20±2.0),　using　actual　domestic　wastewater　as　influent　substrate　and　carbon　fibre　as　biological　carriers　(filling　rate　35%),　the　simultaneous　nitrification　and　denitrification　via　nitrite　was　achieved　in　a　sequencing　batch　biofilm　reactor　under　limited　DO　concentrations.　The　results　of　fluorescence　in-situ　hybridization　(FISH)　demonstrated　that　the　ammonia　oxidizing　bacteria　(AOB)　became　the　dominant　species　in　the　nitrification　process　under　limited　DO　concentration.　During　the　anoxic　and　the　initial　aerobic　process,　the　external　COD　was　taken　up　and　converted　to　polyhydroxyalkanoate　(PHA),　which　can　be　used　as　internal　carbon　sources　for　the　following　denitrification.　DO=0.5　mg/L,　the　NH4+-N　removal,　TN　removal　and　simultaneous　nitrification　and　denitrification　efficiency　was　more　than　95%,　80.4%　and　77.9%,　respectively.　The　concentration　of　NOx--N　in　the　effluent　was　less　than　10　mg/L　and　mainly　in　the　form　of　NO2--N.　When　the　DO　concentration　was　controlled　at　2.0,　1.2　and　0.5　mg/L,　the　N2O　emission　was　1.38,　2.39　and　1.65　mg/L,　respectively.　Both　the　aerobic　denitrification　process　of　AOB　and　the　denitrification　of　NOx--N　with　PHA　as　carbon　source　at　the　presence　of　lower　oxygen　can　lead　to　the　release　of　N2O.　The　long　time　of　lower　DO　conditions　promoted　AOBs　competitive　advantage,　formed　a　micro-environment　for　denitrifiers,　reduced　the　COD　degradation　rate,　and　provided　an　external　carbon　source　as　an　electronic　donor　for　denitrification,　thereby　reducing　N2O　emissions.　©　All　Right　Reserved.