Abstract: 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.