Abstract: Partial nitritation was quickly begun by inoculating aerobic flocculent sludge and half partial nitritation was achieved. The changes in the microbial communities were studied during the process of step cooling. The results show that when controlling ρ(DO) to 1.2-1.5 mg·L^-1, pH to 7.6-7.7, and temperature to (30±2) ℃, partial nitritation could be achieved on the 16th day. Subsequently, half partial nitritation was controlled to stabilize the effluent NO₂^--N/NH₄⁺-N around 1.32. Then a stepwise cooling was set (28 ℃→25 ℃→20 ℃→15 ℃) simultaneously to an increase in ρ(DO) to 1.4-1.5 mg·L^-1 and an increase of pH to 7.8-7.9 to keep the reactor stable at low temperatures. It showed that even at 15 ℃, the NH₄⁺-N removal rate (NRR) and the NO₂^--N accumulation rate (NAR) averaged 59.7% and 80.2%, respectively. According to 16S rDNA high-throughput sequencing tests, the relative abundance of the ammonia oxidizing bacteria (AOB) Nitrosomonas increased from 0.09% in inoculated sludge to 5.20% at 15 ℃, about 58-fold increase. The relative abundance of the nitrite oxidizing bacteria (NOB) Nitrospira and Nitrobacter decreased from 0.90% and 0.98% in the inoculated sludge to 0.33% and 0.05% at 15 ℃, respectively. Thus the number of AOB increased significantly and NOB were effectively inhibited.