Low C/N domestic sewage was treated by an A/O-biological aerated filter (BAF) system at low temperatures (11-14℃). The characteristics of pollutant removal, the ratio of denitrifying phosphorus to nitrogen (ΔPO/ΔNON) and effects of aeration flow and effective packing height on nitrification in BAF were studied. The results showed that when the average influent concentrations of COD, NH-N, TN and PO were 193.1, 58.6, 60.3 and 5.1 mg·L respectively, their effluent concentrations were 46.3, 2.5, 13.4 and 0.3 mg·L respectively, which met the first level A criteria specified in the discharge standard of pollutants for municipal wastewater treatment plant (GB 18918-2002). The linear fitting of ΔPO/ΔNO-N was between 0.47 and 1.75. The normal distribution of mathematical statistics was applied-and the average standard deviation for ΔPO/ΔNO-N were 1.20 and 0.29 respectively. When the aeration flows were 60 L·h and 100 L·h, the effluent concentration of NH-N was less than 5.0 mg·L, corresponding to the effective packing heights in the BAF of 1.8 m and 1.0 m respectively. However, when the aeration flow was increased to 120 L·h, the air-water flow led to biofilm detachment, which caused the effluent concentration of NH-N to increase beyond 5.0 mg·L.