Slow　sand　filters　(SSFs)　have　been　increasingly　applied　to　rainwater　purification　in　recent　years,　but　the　response　of　SSFs　to　fluctuating　rainfall,　as　well　as　the　biofilm-　and　water-phase　microecology　in　SSFs　are　still　poorly　understood.　This　study　systematically　evaluated　the　rainwater　purification　performance　of　SSFs　and　compared　the　bacterial　community　structure,　assembly　processes　and　molecular　ecological　interactions　between　the　biofilm　and　water　phases.　The　activated　carbon　and　activated　alumina　filters　exhibited　the　best　performance　for　NH4+-N　(18.82%similar　to　64.00%)　and　TP　(＞90%)　removal,　respectively.　As　the　filtration　rate　increased　from　0.1　m/h　to　0.3　m/h,　the　rainwater　purification　efficiencies　of　the　three　SSFs　deteriorated　significantly,　with　the　enrichment　of　Tolumonas,　Desulfovibrio　and　Sulfurospirillum,　and　reduction　in　Klebsiella　and　Enterobacter.　The　community　diversity　of　biofilm　phase　was　significantly　higher　than　that　of　water　phase,　and　filtration　rate　was　identified　as　a　key　factor　in　shaping　the　bacterial　community　in　both　phases.　The　interactions　of　filtration　rate　and　water　quality　displayed　the　best　and　significant　(p　＜　0.01)　explanation　for　microbiome　shift,　with　the　higher　values　in　biofilm　phase　(34.70%)　than　in　water　phase　(24.02%).　Bacterial　community　assembly　in　SSFs　was　determined　by　stochastic　ecological　processes,　which　played　a　more　important　role　in　water-phase　communities,　with　86.34%　following　predictions　using　a　neutral　community　model.　The　molecular　ecological　network　of　biofilm　phase　exhibited　more　complexity,　lower　modularity　and　more　cooperative　relationships　than　that　of　water　phase.　Disadvantaged　OTUs　occupied　core　and　notable　positions　in　the　network,　with　the　highest　degree　and　clustering　coefficient.　Different　keystone　species　were　identified　in　biofilm-　(Runella　,　Aquabacterium,　etc)　and　water-　network　(Terrimonas)　respectively,　despite　they　processed　low　relative　abundances　(＜0.1%).　These　results　enhance　the　understanding　of　microecology　in　SSFs,　and　shed　new　lights　on　the　improvement　and　promotion　of　rainwater　biological　treatment　technology.