Biofilms　in　the　drinking　water　distribution　system　(DWDS)　provide　shelter　for　pathogens　and　antibiotic　resistance　genes　(ARGs).　However,　how　biofilms　alter　the　microbiome　and　antibiotic　resistome　in　tap　water,　as　well　as　the　precise　quantitative　evaluation　of　their　health　risks,　remains　unclear.　Herein,　biofilm　reactors　supplied　with　municipal　drinking　water　were　operated　for　120　days.　Metagenomic　sequencing　identified　significant　differences　in　microbial　compositions　among　the　biofilms,　influent,　and　effluents.　A　total　of　69-305　ARGs　were　detected　in　this　DWDS,　and　ARG　abundances　increased　in　the　biofilms　(0.246-1.576　cpc)　and　effluents　(0.309-0.503　cpc)　compared　to　the　influent　(0.131　cpc).　Metagenomic　assembly　pinpointed　potential　pathogenic　ARG　hosts　such　as　Acinetobacter,　Pseudomonas,　and　Escherichia.　The　co-occurrence　of　ARGs　and　mobile　genetic　elements　indicated　potential　mobility,　which　was　further　supported　by　transformation　assays　demonstrating　gene　transfers　at　a　frequency　of　10-6.　Furthermore,　source　tracking　revealed　that　biofilms　contributed　high　proportions　(19　%-34　%)　to　the　ARG　profiles　of　effluents.　The　ARG　risk　scores　increased　from　the　influent　(20.39)　to　the　effluents　(39.85-55.50),　with　highest　level　(55.50)　in　the　cast　iron　effluent.　Overall,　this　study　provides　novel　insights　into　the　impacts　of　biofilm　growth　on　the　microbiome　and　antibiotic　resistome　in　tap　water,　along　with　their　potential　health　risks　in　the　DWDS.