In　order　to　analyze　the　disinfection　by-products　in　reclaimed　water,　a　rapid　analyzing　method　of　the　THMs　(Trihalomethanes),　a　types　of　the　disinfection　by-product,　has　been　developed　by　using　static　headspace　sampling,　capillary　gas　chromatography,　mass　spectrometry　and　ion　selective　quantitative　analysis　through　the　optimization　of　headspace　sampling,　chromatography　and　mass　spectrometry　testing　conditions.　The　results　show　that　the　fragment　ions　(A)　of　connecting　two　halogen　atoms　and　their　isotope　ions　(A+2,　A+4)　are　of　much　higher　peak　abundance　than　other　mass　ions　in　the　mass　spectrum　of　four　THMs　(including　chloroform,　dichlorobromomethane,　dibromochloroethane,　bromoform).　It　is　highly　specific,　selective　and　representative　to　choose　the　character　ions　for　the　THMs　quantitative　analysis,　and　at　the　same　time　the　method　sensitivity　is　also　improved.　An　external　standard　method　is　used　to　quantitively　analyze　the　THMs　in　the　reclaimed　water　based　on　the　sum　of　selected　ion　monitoring　(∑SIM)　of　m/z　83,　85,　87　for　a　chloroform　and　a　dichlorobromomethane,　m/z　127,　129,　131　for　dibromochloroethane,　and　m/z　171,　173,　175　for　bromoform.　The　method　recoveries　range　from　83.4%　to　112.2%,　the　relative　standard　deviations　(RSD)　are　from　3.5　percent　to　8.2　percent,　and　the　detection　limits　are　less　than　0.21　μg/L.　THMs　concentration　of　the　reclaimed　water　is　above　100　μg/L.