Abstract: A reactor which combined traditional anoxic aerobic process with biofilm was applied to treat simulated domestic sewage, of which the optimum operating condition was determined by adjusting the hydraulic residence time (HRT), carbon/nitrogen ratio (C/N) and reflux ratio of nitrating liquid. Besides, the operation efficiency of the reactor was investigated by adding ammonia nitrogen removal agent and denitrifying agent. The results showed that the reactor tended to be stabilized in 30 days after startup. When the HRT was 24 h, C/N was 4, and reflux ratio was 200%, the reactor operating effect was the best. The average removal rates of chemical oxygen demand (COD), ammonia nitrogen ( ${\mathrm{NH}}_4^{+}$ -N) and total nitrogen (TN) was 80.3%, 79.9% and 47.8%, respectively. The capacity of the reactor to remove ammonia nitrogen was strengthened after adding microbial agent. Accordingly, the average removal rates of the three pollutants turned into 79.5%, 90.2% and 56.6%, respectively, among which the maximum removal rate of TN could be increased to 70.0%. Besides, the tolerance of reactor to influent load was improved to a certain extent. Furthermore, the high throughput sequencing (HTS) analysis showed that Proteobacteria and Bacteroidetes were the main functional microorganisms in the original sludge, of which the total relative abundance ratios were 77.8%. During the operation of the reactor system, the relative abundance of the two phyla showed a tendency of decreasing first, then increased and tended to be stable. After the addition of microbial agent, the species number and diversity of microorganisms in the anoxic pool increased significantly, and the relative abundance of the two phyla in the aerobic pool reached 92.3%.