This　study　investigated　the　water　purification　function　and　mechanism　of　biofilm　in　storage　tank,　with　a　view　to　using　it　as　a　pretreatment　unit　for　rainwater　cleaner　production.　Shortening　the　hydraulic　retention　time　(HRT)　of　storage　tank　from　12　to　4　h　improved　the　pollutants　removal　performance　and　reduced　the　suspended　bacteria　counts.　The　accumulation　of　abundant　taxa　and　succession　of　rare　taxa　were　observed　with　biofilm　development.　Positive　correlations　within　and　across　different　bacterial　taxa　were　dominant　in　the　network,　and　some　rare　genera　(Ralstonia　and　Micropruina)　were　identified　as　hub　bacteria.　Candidatus　Nitrospira　nitrosa　and　Nitrospira　sp.　ENR4　were　two　identified　complete　ammonia　oxidizers.　Denitrifying　bacteria　tended　to　enrich　and　formed　more　complex　interactions　over　time.　The　main　nitrogen　metabolism　pathways　may　be　ammonia　assimilatory,　complete　denitrification　and　dissimilatory/assimilatory　nitrate　reduction.　HRT　was　negatively　correlated　with　most　dominant　genera,　and　contributed　20.35%　to　the　variation　of　functional　taxa.　This　study　highlights　the　self-purification　function　and　micro-ecology　of　storage　tank,　and　provides　a　new　insight　for　its　role　in　rainwater　cleaner　production　process.