The　succession　of　the　microbial　community　in　the　biofilm　was　studied　in　three-stage　catching　bed　reactor　system　using　PCR-DGGE　and　cloning　technology.　The　homology　of　the　microbial　community　in　the　biofilm　was　analyzed　and　the　phylogenetic　trees　were　constructed.　Meanwhile,　the　contributions　of　microbial　community　in　the　biofilm　on　removal　rates　of　COD　and　total　nitrogen　in　the　simultaneous　nitrification　denitrification　(SND)　system　were　discussed.　The　results　showed　that　the　microbial　community　in　the　biofilm　appeared　to　have　great　evolution　and　high　diversity　during　running.　Furthermore,　according　to　denaturing　gradient　gel　electrophoresis　(DGGE)　analysis,　the　dominant　bacterial　community　in　the　reactor　could　be　mainly　consisted　of　5　groups:　β-proteobacteria,　γ-proteobacteria,　unclassified　bacteria,　α-proteobacteria　and　Actinobacteria.　In　addition,　β-proteobacteria　had　an　advantage　in　numbers,　which　played　an　important　role　in　the　degradation　of　COD　removal.　Nitrosomonas　sp.　and　uncultured　Nitrospirae　bacterium　which　could　oxidize　ammonia　and　nitrite　in　the　system　were　also　found,　Pseudomonas.　stutzeri　which　was　recognized　as　aerobic　denitrifying　bacteria　was　detected　in　the　reactor.