In order to provide some support for economically and effectively resolving the problem of water pollution around the islands in the South China Sea, biological nitrogen removal (BNR) performance and biotechnological characterization of the FSBR filled with abandoned coral sand were investigated. The results showed that the TN removal efficiency of the FSBR with a good performance of simultaneous nitrification and denitrification (SND) was 74.68±6.49% during the stable operation. The total biomass and removal efficiency of pollutants in the three reaction regions all decreased along the flow direction. Therefore, the first region was the key area for SND reaction and TN removal. In the first region, the nitrification process was the result of the combination of autotrophic nitrification and heterotrophic nitrification. Meanwhile, aerobic denitrification played a key role in the process of denitrification. In the region, the most denitrifying bacteria belonged to Proteobacteria, in which unclassified_ f _Comamonadace, Acidovorax and Dokdonella were the major aerobic denitrifying bacteria.