Based on 16S rRNA genes-Illumina MiSeq high-throughput sequencing, this study is to investigate the community and structure of characteristic microbial communities related to nitrification under four C/N ratios (0、5、10、15) settled in four parallel laboratory-scale sequencing batch reactors (SBRs, denoted as R₀、R₅、R₁₀、R₁₅) to better understand the biological mechanisms of how does C/N ratio influence the nitrification. Results indicated that effective removal of ammonia nitrogen (removal efficiency > 95%) and COD (removal efficiency > 90%) were achieved in four C/Ns, and TN has also been reduced to varying degrees. In addition, C/N ratio has an important impact on the diversity, community and function of the bacterial communities. The largest α-diversity values of Chao1 (922), ACE (1232.4), Shannon (6.76) and Simpson (0.96) were achieved in R₀, which represents with highest richness and diversity of bacterial community, while the lowest was achieved in R₅. 9 phyla such as Proteobacteria, Bacteroidetes, Cyanobacteria and so on were found to be major groups with higher relative abundance at the phylum. The most dominant bacterial is affiliated with Proteobacteria comprised 40.7%~65.2% in the four sludge samples. The relative abundance of key microorganisms as Nitrosomonadaceae and Nitrospira related to nitrification decreased sharply with increasing C/N. A total of 34 groups of microorganisms with significant differences were obtained based on LEfSe analysis, thereby key biomarkers of the microflora were obtained at the microbiological classification level under each C/N condition.