To　ensure　the　biosafety　of　the　domestic　hot　water　system　(DHWS)　in　the　case　of　residual　chlorine　exhaustion　and　biofilm　exfoliation,　the　secondary　disinfection　effects　of　chlorine　alone　or　in　combination　with　ultraviolet　(UV)　on　microorganisms　and　its　impact　on　biofilm　bacterial　community　were　investigated.　A　biofilm　annular　reactor　was　used　to　simulate　DHWS　conditions　on　a　lab-scale　and　reproducibly.　The　results　showed　that　even　in　the　case　of　biofilm　exfoliated　into　domestic　hot　water　seriously,　the　chlorine　combined　with　UV　(chlorine-UV)　disinfection　could　achieve　high-level　inactivation　of　suspended　bacteria　in　a　short　time.　Chlorine-UV　disinfection　effectively　reduced　the　diversity　of　the　bacterial　community　and　affected　bacterial　community　structure.　It　decreased　the　relative　abundance　of　pathogenic　bacteria,　including　Legionella,　Staphylococcus.　Chlorine-UV　disinfection　is　suitable　for　the　secondary　disinfection　of　DHWS,　which　can　ensure　biosafety　and　effectively　reduce　bacterial　contamination.