The method of determination of trace 2MIB and GSM in water was established by headspace solidphase microextraction (HS/SPME) coupled with gas chromatography/tandem mass spectrometry, and the conditions of HS/SPME were optimized. After the instrument conditions were optimized, the linear relationship between peak area and concentration of 2MIB and GSM was good enough (r≥0.999 4) from 1.00 to 50.0 ng/L. When the standard sample of 1.0 ng/L was determined 7 times in parallel, the RSDs of 2MIB and GSM were 7.36% and 7.75% respectively. Their detection limits were 0.226 and 0.239 ng/L, and their quantification limits were 0.904 and 0.956 ng/L, respectively. Two actual water samples were determined and spiked with 2.00 and 20.0 ng/L respectively, and the recovery rates of 2MIB and GSM were 87.0%~97.0% and 85.2%~112%. The result shows that the proposed method is easily operational, highly accurate, well reproducible, highly sensitive and no need of organic solvents. And it is suitable for the determination of trace 2MIB and GSM in water.