Abstract: By treating real domestic sewage which is low ρ(C)/ρ(N) in sequencing batch reactor (SBR), the cause of filamentous sludge bulking and the effect of controlling filamentous sludge bulking under low dissolved oxygen with different operation patterns were investigated.Results showed that when the dissolved oxygen (DO) value was 0.5 mg/L in anoxic/oxic process (A/O) , filamentous sludge bulking happened and H. hydrossis was the dominant filamentous bacteria in filamentous bulking sludge. During A/O pattern in SBR, sludge volume index (SVI) has maintained beyond 300 mL/g when organic loading (F/M) was 0.83 kg/(kg·d) and the ρ(DO) was about 2 mg/L, the sludge bulking could not be effectively controlled. The result of fluorescence in situ hybridization (FISH) manifested that the main filamentous bacteria was also H. hydrossis filamentous bacteria. In this period, average removal rates of total nitrogen (TN) and COD were 31.17% and 65.04%, respectively. Then during aerobic pattern, when F/M was 0.37 kg/(kg·d) , the ρ(DO) was also about 2 mg/L, the sludge bulking has been controlled effectively and SVI has remained below 150 mL/g. In this stage, the removal rate of NH₄⁺-N reached 100% nearly, the ρ(NH₄⁺-N) in effluent was almost 0. Average removal rates of TN and COD improved to 70.7% and 73.9%, respectively. The average ρ(TN) and ρ(COD) in effluent were 18.4 and 46.9 mg/L, respectively, close to the national level A emission standard.