To　investigate　macroscopic　performance　and　microcosmic　ecosystem　of　CANON　process　in　low　ammonium　concentration　at　room　temprature,　a　steady-operation　CANON　reactor　was　achieved　in　different　ammonium　concentrations　by　adjusting　aeration　and　hydraulic　retention　time　(HRT),　and　the　effect　of　ammonium　concentration　on　the　abundance　and　community　structure　of　functional　bacteria　was　analyzed　using　PCR-DGGE　and　real-time　PCR.　The　results　indicated　a　high　TN　removal　loading　of　over　1.0　g·(L·d)-1　in　relatively　high　ammonium　which　was　significantly　reduced　with　the　ammonium　concentration　of　100　mg·L-1.　The　community　structure　of　ammonium　oxidizing　bacteria　(AOB)　changed　sharply　with　the　decrease　of　ammonium　concentraion,　which　was　not　the　same　as　anaerobic　ammonium　oxidizing　bacteria　(ANAMMOX　bacteria).　Besides,　the　abundance　of　the　two　functional　groups　of　bacteria　reduced　while　the　population　of　nitrite　oxidizing　bacteria　(NOB)　rose　with　the　decrease　of　ammonium,　which　might　be　the　main　reason　for　the　reduction　of　nitrogen　removal　efficiency.　Consequently,　some　strategies　are　needed　to　reduce　the　loss　of　AOB　and　ANAMMOX　bacteria　and　inhibit　the　growth　of　NOB　so　as　to　maintain　the　nitrogen　removal　performance　in　low　ammonium　concentration.