Abstract: In order to analyze the disinfection by-products in reclaimed water,a rapid analyzing method of the THMs (Trihalomethanes),a types of the disinfecti on by-product,has been developed by using static headspace sampling,capillary gas chromatography,mass spectrometry and ion selective quantitative analysis th rough the optimization of headspace sampling,chromatography and mass spectromet ry 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 abu ndance than other mass ions in the mass spectrum of four THMs (including chlorof orm,dichlorobromomethane,dibromochloroethane,bromoform).It is highly specif ic,selective and representative to choose the character ions for the THMs quant itative 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 recl aimed water based on the sum of selected ion monitoring (ΣSIM) of m/z 83,85,8 7 for a chloroform and a dichlorobromomethane,m/z 127,129,131 for dibromochlo roethane,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 concentra tion of the reclaimed water is above 100 μg /L.