In　order　to　investigate　the　inactivation　efficiency　of　ultraviolet　(UV)　reactor　on　the　secondary　wastewater,　the　UV　inactivation　experiment　on　the　secondary　effluent　of　wastewater　treatment　plant　was　carried　out.　Different　water　layer　thickness,　different　radiation　time　and　photoreaction　effect　were　experimented.　Four　closed　disinfection　reactors　were　adopted,　and　the　water　layer　thicknesses　were　10.5　mm,　20　mm,　31　mm　and　40.5　mm　respectively.　The　inactivation　efficiencies　of　the　four　reactors　were　detected　in　9　radiation　times　which　were　3　s,　5　s,　10　s,　20　s,　30　s,　40　s,　50　s,　70　s　and　100　s.　And　we　investigated　the　photoreaction　effect　with　the　two　reactors　of　20　mm　and　31　mm　water　layer　thicknesses.　The　investigation　showed　that　thinner　water　layer　led　to　few　surviving　bacteria　in　the　same　radiation　time,　and　it　was　more　apparent　in　short　radiation　time;　longer　radiation　led　to　few　surviving　bacteria　in　the　same　water　layer　thickness;　the　microorganisms　in　the　water　samples　would　become　photo-reactivate　after　UV　inactivation.　We　could　conclude　that　the　inactivation　ratio　of　organisms　in　wastewater　may　be　above　99.9%　after　being　radiated　by　UV.　Radiation　time　and　water　layer　thickness　are　two　main　factors　of　influencing　inactivation　effect.　Longer　radiation　time　and　less　water　layer　thickness　led　to　better　disinfection　effect　of　wastewater　by　UV.　To　some　extent,　photo-reactivation　also　influences　inactivation　effect,　supplying　more　ultraviolet　dose　may　reduce　the　reviving　ratio　of　microorganisms.