Two　main　problems　in　partial　nitrification　at　low　temperature　are　its　difficult　start-up　and　easy　inhibition,　and　this　issue　significantly　limits　the　broad　application　of　advanced　partial　nitrification　technology.　To　promote　rapid　culturing　and　stabilization　of　partial　nitrification　biofilms,　two　experiments　were　carried　out　to　investigate　the　influence　of　C/N　on　partial　nitrification　through　ratio　control　(R,　dissolved　oxygen　(DO)/effluent　total　ammonia　nitrogen　(TAN(eff)))　in　a　lab-scale　moving　bed　biofilm　reactor　(MBBR)　at　12　degrees　C.　One　experiment　looked　at　the　effect　of　C/N　on　the　stable　operation　of　partial　nitrification　at　12　degrees　C,　which　inoculated　a　mature　biofilm　with　good　partial　nitrification　performance;　the　other　looked　at　the　effect　of　C/N　on　the　biofilm　cultivation　and　start-up　of　partial　nitrification　at　12　degrees　C,　which　inoculated　activated　sludge　and　Pall　rings.　As　for　the　startup　experiments,　only　the　C/N　ratio　was　investigated,　as　the　DO/TAN(eff)　ratio　was　kept　constant.　The　effect　of　C/N　on　the　mature　partial　nitrification　biofilm　showed　that　the　increase　of　C/N　could　increase　the　resistance　of　partial　nitrification　to　high　DO.　The　nitrite　accumulation　ratio　(NAR,　(NO2--N/(NO2--N　+　NO3--N)　x　100%))　increased　from　40.41%　to　98.78%　when　R　was　0.25　and　C/N　increased　from　0　to　1.　Meanwhile,　when　C/N　increased　to　2　and　the　R　value　slightly　increased　to　0.3,　partial　nitrification　failed.　The　penetration　of　DO　in　the　biofilm　indicated　that　the　increase　of　C/N　enhanced　the　mass　transfer　resistance　of　DO,　but　it　was　similar　when　C/N　was　1　and　2.　Adding　an　appropriate　amount　of　C/N　can　reduce　the　negative　effect　of　partial　nitrification　damage　caused　by　DO/TAN　enlargement.　The　start-up　experiment　showed　that　C/N　=　1　and　C/N　=　2　were　essential　for　culturing　biofilms　and　achieving　partial　nitrification　at　12　degrees　C,　which　can　be　successfully　achieved　(NAR　was　85%　and　96%)　on　the　56th　and　52nd　days.　Neither　C/N　=　0　nor　C/N　=　3　could　effectively　achieve　the　partial　nitrification.　The　increase　of　C/N　will　reduce　the　proportion　of　nitrifying　bacteria　in　the　biofilm,　which　is　harmful　to　the　partial　nitrification.　This　will　also　lead　to　the　decrease　of　protein/polysaccharide　(PN/PS)　and　affect　the　stability　of　the　biofilm.