Biofilm　in　secondary　water　supply　system　(SWSS)　may　affect　tap　water　biological　stability.　The　growth　of　biofilm　and　the　diversity　of　biofilm　microbial　community　are　partly　influenced　by　organic　matter　content.　In　this　paper,　the　impact　of　organic　matter　content　on　biofilm　bacteria　growth　and　biofilm　density　was　studied.　The　diversity　and　variation　process　of　biofilm　microbial　community　were　also　analyzed　by　Metagenomic　sequencing.　Biofilm　bacteria　growth　and　compactness　were　positive　correlate　with　organic　matter　content.　Biofilm　grew　with　tap　water　of　low　organic　matter　content　(permanganate　oxygen　consumption,　viz.,　CODMn　＜3　mg/L)　was　of　higher　diversity　level　than　that　in　the　condition　of　CODMn　＞3　mg/L.　It　may　because　of　the　proportion　of　Proteobacteria,　Firmicutes　and　Bacteroidetes　changed　in　low　CODMn　concentration　condition　in　comparison　to　that　in　high　CODMn　concentration　condition.　Proteobacteria　and　Firmicutes　were　dominant　at　phylum　level　in　biofilm　in　different　organic　matter　content　in　this　experiment.　Pathogenic　bacteria　including　Pseudomonas,　Acinetobacter,　Ledionella　and　Flavobacterium　could　grow　with　tap　water　that　met　the　Chinese　drinking　water　standards　(CODMn　＜3　mg/L).