NOVELTY - Starting sewage continuous flow short-cut nitrification under room temperature and low carbon-to-nitrogen (C/N) ratio, comprises: (i) providing an anaerobic/anoxic/oxic (A2O) reactor divided into 3 sections i.e. aerobic segment-1, aerobic segment-2 and aerobic segment-3 along the flow direction; (ii) passing 2.5-3 g/l nitrification sludge i.e. municipal sewage treatment plant A2O process secondary sedimentation tank sludge into the reactor; and (iii) maintaining certain operation conditions until the effluent nitrification rate is more than 85% and elutriation of nitrite-oxidizing bacteria.    USE - The method is useful for starting sewage continuous flow short-cut nitrification under room temperature and low C/N ratio (claimed).    ADVANTAGE - The method is simple and convenient.    DETAILED DESCRIPTION - Starting sewage continuous flow short-cut nitrification under room temperature and low C/N ratio, comprises: (i) providing an anaerobic/anoxic/oxic (A2O) reactor divided into 3 sections i.e. aerobic segment-1, aerobic segment-2 and aerobic segment-3 along the flow direction, where the aerobic segment-3 is connected back to anoxic area for refluxing nitrification liquid; (ii) passing 2.5-3 g/l nitrification sludge i.e. municipal sewage treatment plant A2O process secondary sedimentation tank sludge into the reactor; (iii) maintaining: water temperature to 23-25 degrees C, the ratio of nitrite-oxidizing bacteria in anaerobic zone, hypoxia zone, aerobic segment-1, aerobic segment-2 and aerobic segment-3 to 1:2:1:1:1, mass concentration of C/N to 3.5-4, chemical oxygen demand concentration of 340-385 mg/l, ammonia-nitrogen concentration of 80-92 mg/l, total nitrogen concentration of 93-112 mg/l, total phosphorus concentration of 5-6 mg/l and pH of sewage is 7-7.5, maintaining water flow quantity of 13-14 l/height, simultaneously controlling the dissolved oxygen concentration in aerobic zone to 2-3 mg/l, hydraulic retention time for 8-8.3 hours, sludge concentration of 2.5-3 g/l, sludge reflux ratio of 70-80% and nitrification liquid reflux ratio of 200-250%, feeding sludge-free water into the reactor and operating continuously for 48 hours, recovering activated sludge, controlling initial sludge for 3 days and lower sludge for 8-9 days, maintaining the operation condition until the effluent nitrification rate is more than 85% and elutriation of nitrite-oxidizing bacteria; (iv) adding 20-25% polypropylene columnar suspended filler into aerobic zone-1 and aerobic zone-2; and (v) carrying out continuous flow enrichment culture of ammonia oxidizing bacteria by maintaining the ratio of ammonia-oxidizing bacteria in anaerobic zone, hypoxia zone, aerobic segment-1, aerobic segment-2 and aerobic segment-3 to 1:2:1:1:1, using 3 domestic sewage, maintaining inlet rate of 13-14 l/hour with hydraulic retention time of 8-8.3 hour, sludge concentration of 2.5-3 g/l, sludge recycle ratio of 70-80%, nitrification liquid reflux ratio of 200-250% and extending sludge retention time for 11-12 days, reducing dissolved oxygen concentration in the aerobic zone to provide dissolved oxygen concentration in the aerobic segment-1, aerobic segment-2 and aerobic segment-3 to be 1.5-1.7 mg/l, 0.7-1.2 mg/l and 0.4-0.6 mg/l, maintaining reduced ammonia concentration in the reactor effluent to be less than or equal to 2 mg/l, maintaining the operation condition until the effluent nitrification rate is stable at 80% to complete continuous flow short-cut nitrification startup.