Abstract: A method for rapid determination of 4 isothiazolinones in vegetables, including 5-chloro-2-methyl-4-isothiazolin-3-one (CMIT), 2-methyl-4-isothiazolin-3-one (MIT), 1,2-benzoisothiazolin-3-one (BIT), and 2-n-octyl-4-isothiazolin-3-one (OIT), by ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) was proposed. The homogenized vegetable sample (10.0 g) was taken, extracted by ultrasound in 10 mL of acetonitrile for 30 min, and then centrifuged. The residue was extracted again, which was combined with the extraction solution. 5 g of sodium chloride was added. After oscillation and settlement, the extraction solution was blown to near dryness by nitrogen at 40 ℃. The residue was dissolved in 2 mL of 10% (volume fraction, the same below) methanol solution. The obtained solution were purified by HLB solid phase extraction column, which was eluted with 5 mL of 10% methanol solution and 6 mL of methanol. The eluate was blown to near dryness by nitrogen at 40 ℃, and made its volume up to 1 mL with methanol, which was passed through 0.22 μm organic filter membrane. The filtrate was separated on ACQIUTY UPLC BEH SHIELD RP18 column, with a mixture of methanol and water at different volume ratios for gradient elution. Multiple reaction monitoring mode and electrospray ionization positive ion mode were adopted in MS analysis, and matrix matching method was used for drawing the working curves. As shown by the results, linear relationships between the corresponding peak areas and mass concentrations of 4 isothiazolinones were found in definite ranges, with detection limits (3S/N) of 0.5,0.5,0.5,0.025 μg·kg^-1, respectively. Test for recovery was made at 3 concentration levels in different vegetable matrices, giving results in the range of 68.8%-81.3%, with RSDs (n=6) of the determined values less than 9.0%. This method was used for the analysis of 20 vegetable samples, and BIT was detected in only one group of leek samples, with the detected amount of 2.3 μg·kg^-1.