The　experiment　used　the　three-level　anoxic/oxic　(A/O)　coupled　biofilm　reactor　as　the　object　to　investigate　the　effect　of　hydraulic　retention　time　(HRT)　on　the　removal　of　pollutants　and　sludge　characteristics　at　low　temperature.　The　low　C/N　value　municipal　wastewater　was　treated　at　the　temperature　was　10　degrees　C　+/-　1　degrees　C,　the　influent　flow　distribution　ratio　was　25:15:6,　the　total　aeration　was　6　L　min(-1),　the　sludge　residence　time　was　43　+/-　2　d　and　the　sludge　return　ratio　was　50%.　The　reactor　was　operated　under　three　operating　conditions　with　HRT　were　12,　8　and　6　h,　respectively.　The　results　show　that　the　denitrification　effect　was　the　best　when　HRT　was　8　h,　and　the　average　removal　rates　of　chemical　oxygen　demand,　NH4+-N　and　total　nitrogen　were　85.08%　+/-　3.10%,　92.03%　+/-　4.51%　and　72.02%　+/-　3.03%,　respectively.　At　the　same　time,　with　the　decrease　of　HRT,　the　values　of　sludge　volume　index,　the　contents　of　extracellular　polymeric　substances　(EPS)　and　the　values　of　dehydrogenises　activity　of　the　activated　sludge　and　the　EPS　contents　of　the　biofilm　were　increased.　When　the　HRT　was　8　h,　the　aerobic　denitrification　and　aerobic　simultaneous　nitrification　and　denitrification　of　the　coupling　process　were　rationally　utilized,　so　that　the　denitrification　effect　was　better.　The　results　enrich　the　theory　of　denitrification　in　multi-stage　A/O　coupling　biofilm　reactor　under　low　temperature,　and　provide　reference　for　engineering　design　and　application.