In　order　to　analyze　microbial　community　and　phylogenesis　in　nitrosification　biofilm　reactor,　a　partial　stretch　of　the　gene　encoding　the　active-site　polypeptide　of　ammonia　monooxygenase　(amoA)　was　amplified　and　the　gene　libraries　were　constructed.　The　result　of　gene　sequences　and　phylogenetic　analysis　showed　that　Nitrosomonas　eutropha　was　the　predominant　species　in　the　reactor.　Besides,　there　were　also　some　kinds　of　ammonia-oxidizing　microbe　uncultured　in　the　system.　PCR-SSCP　analysis　of　16SrDNA　of　archaebacteria　and　eubacterium　indicated　that　with　the　rising　of　ammonia　oxidation　rate,　the　structure　and　distribution　of　microbial　community　was　influenced　and　the　diversity　of　microbial　communities　decreased,　and　the　decreasing　of　specificity　in　the　reactor　might　be　the　key　factor　for　the　rising　of　ammonia　oxidation　rate.