To　investigate　the　features　of　denitrifying　bacteria　in　a　cathode　of　a　3DBER-S　reactor　and　enhance　the　nitrogen　removal　mechanism　in　low　carbon-to-nitrogen　ratio　conditions,　the　community　structure　of　denitrifying　bacteria　in　cathode　biofilms　was　analyzed　using　a　nirS　gene　clone　library　after　stably　operating　the　reactor　under　TOC/TN=0.36　in　the　influent.　The　dominant　class　was　β-proteobacteria,　which　accounted　for　59.22%　of　all　denitrifying　bacteria.　Among　the　denitrifying　bacteria,　the　most　highly　represented　species　were　similar　to　Azoarcus　tolulyticus　and　Magnetospirillum　magneticum,　which　accounted　for　44.74%　and　21.05%　of　species,　respectively.　The　total　proportion　of　species　that　were　Sulfuricella　denitrificans,　Dechlorospirillum　sp.　and　Thauera　sp.　reached　17.11%;　these　three　kinds　of　bacteria　use　elemental　sulfur　or　hydrogen　as　an　electron　donor　for　denitrification.　In　conclusion,　nitrogen　removal　depended　on　heterotrophic　denitrification,　hydrogen　autotrophic　denitrification,　and　sulfur　autotrophic　denitrification.　Not　only　did　it　reduce　the　consumption　of　the　organic　carbon　source,　but　it　also　maintained　the　pH　balance　of　the　system.　Thus,　a　stable　and　efficient　nitrogen　removal　system　can　be　maintained　in　low　carbon-to-nitrogen　ratio　conditions.　©　2016,　Science　Press.　All　right　reserved.