Effects of different C/N (NO2–N) ratios on nitrogen removal and microbial community structure were investigated using an anaerobic baffled reactor (ABR). Results indicated that the C/N ratio exerted an important effect on nitrogen removal in the anammox-coupling-denitrification process associated with the ABR. When the C/N ratio was 1.29, the ABR could achieve the highest total nitrogen (TN) removal efficiency of 99.9%. Most of TN was removed in the 1st and 2nd compartment, accounting for about 81.0–97.6% of total TN removal. The nitrogen removal resulted from the interaction among anammox, heterotrophic denitrificans, and other microbes within the ABR. The contribution of anammox to nitrogen removal varied from 6.8% to 32.4%. High-throughput MiSeq sequencing analyses revealed that the C/N ratio was one of the most important factors regulating the microbial community structure, and the predominant phylum changed from Proteobacteria to Chloroflexi with the elevated C/N ratio. In addition, the Candidatus Brocadia was the major anammox bacterium, and its percentage varied from 1.0–2.9% at day 9 to 2.8–9.1% at day 46.