Abstract: In this study,a headspace gas chromatography-mass spectrometry(HS-GC /MS)method for simultaneous determination of cracking C₉(including α-methylstyrene,o-methylstyrene,m-methylstyrene,p-methylstyrene,β-methylstyrene,dicyclopentadiene,ninhydrin,α-methylnaphthalene and β-methylnaphthalene)in water sample was established.Briefly,10 mL seawater was sealed in the headspace bottle and vibrated under 90 ℃ for 10 min to obtain gas-liquid dynamic equilibrium.Nine target compounds were separated by DB-5MS capillary column(30 m×0.25 mm×0.25 m),and quantified by internal standard method.The method had good linear relation in the range of 0.50~10 μg·L^-1,with a correlation coefficient greater than 0.998.The detection limit of the compounds in seawater was 0.500~1.00 μg·L^-1.When the concentration was 1.00~10.0 μg·L^-1,the average standard recovery of each compound was 82.4%~106.6% and the relative standard deviation was 3.73%~9.41%.This simple,rapid,sensitive and accurate method was suitable for the simultaneous detection of various kinds of cracking C₉ in seawater,which provided a reference for the determination of cracking C₉ in the emergency monitoring of marine pollution accidents.