A pilot-scale A²O process was carried out to treat sewage in a wastewater treatment plant in Chaohu City, aiming to save energy. The results showed that when the dissolved oxygen (DO) decreased from 2mg/L to 0.5mg/L, the removal rates of COD and ammonia were 80% and 90%, respectively, meaning the removal performance was not affected, and no nitrite accumulation occurred, in the meantime, the removal rate of TN increased significantly, from 15% to 44%. Under low DO condition,the decrease of temperature mainly affected the TN removal rate, from 44% in summer to 29% in winter, whereas the removal rate of ammonia was still above 90%. Due to the shortage of carbon source, the nitrogen removal rate was only 29% under low temperature, resulting in a high nitrate concentration in the effluent. Under long-term low DO conditions, the dominant microbial species of AOB and NOB were Nitrosomonadaceae and Nitrospira, with relative abundance of 2.33% and 6.40% respectively. NOB is superior to AOB in both quantity and kinetic performance. At the same time, Denitratisoma was found with the relative abundance of 1.59%, which was considered as aerobic denitrifying bacteria. These findings provide theoretical and practical support for nitrogen removal from municipal wastewater under low DO conditions.