Abstract: GC-MS was applied to the determination of 25 volatile organic compounds in water and its application in tracing sources of sewage treatment plant impact emergency was studied.Linear relationships between values of peak area and mass concentration of the 25 volatile organic compounds were kept in definite ranges,with detection limits (3.143s) in the range of 0.6-5.0 μg·L-1.On the base of blank sample,test for recovery was made by standard addition method;values of recovery found were in the range of 91.6%-102%,with RSD′s (n=6) in the range of 2.7%-4.6%.In one case of the wastewater treatment plant impact,the pollution factor was identified to be dichloromethane by sampling the sewage pipe network and analyzing sewage samples.Then the pollution source was quickly identified in accordance with the changes of dichloromethane concentration to ensure the quality of the sewage plant outlet water.