To　ensure　the　safety　of　water　in　the　process　of　translation　in　long　distance　raw　water　diversion　pipe,　the　damagemant　and　recovery　of　purification　by　added　chlorine　to　Biofilms　Annular　Reactor(BAR)　which　simulated　the　pipe　was　investigated.　The　results　showed　that　the　prechlorination　can　damage　the　pipe　bioflim,　lower　the　removal　rate　of　NH4+-N,　NO2--N,　CODMn,　TP,　and　the　effluent　of　concentration　of　CODMnand　TP　was　elevated　in　short　term　and　NO2--N　concentration　was　rised　in　long　term　after　prechlorination.　The　effluent　concentration　of　NO2--N　can　be　reached　7　times　of　the　influent.　The　recovery　of　removal　of　NH4+-N　was　earlier　2　days　than　that　of　NO2--N,　and　NH4+-N　and　NO2--N　could　be　completely　repaired　on　8　dsys　and　10　days,　respectively.　The　removal　of　CODMn　and　TP　can　be　completely　repaired　on　8　days.