Abstract: A method for determination of residues of 11 neonicotinoid insecticides, including fluazinam, fipronil, imidacloprid, thiamethoxam, thiacloprid, epoxystrobin, thiamethoxam, imidacloprid, chlorothiazide, acetamiprid and flupyradifurone in fish by liquid chromatography-tandem mass spectrometry (LC-MS/MS) with QuEChERS purification was proposed. 4.00 g of fish sample was taken and placed in a 50 mL-polypropylene centrifuge tube, and 20 mL of 20% (volume fraction) acetonitrile solution containing 0.4% (volume fraction) acetic acid was added. The mixture was vortexed for 1 min, and sonicated for 20 min. 1 g of magnesium sulfate was added, and the mixture was vortexed for 1 min and centrifuged for 10 min. 10 mL of supernatant was taken and placed in a 15 mL-test tube, and blown to near dryness by nitrogen at 35 ℃. 50 mg of N-propylethylenediamine, 40 mg of C₁₈ powder, and 2 mL of 25% (volume fraction) acetonitrile solution were added. The mixture was vortexed for 2 min, and 1 mL of purified solution was passed through a 0.22 µm organic filter membrane. 11 neonicotinoid insecticides in the filtrate were determined by LC-MS/MS. ZORBAX SB-AQ column was used as the stationary phase and the mixed solution composed of 0.1% (volume fraction) formic acid solution and acetonitrile at different volume ratios was used as the mobile phase for gradient elution．Electrospray ion (ESI) source was used for mass spectrometry. The targets were analyzed by multiple reaction monitoring (MRM) mode in positive ion (ESI⁺) scanning mode. Working curves were drawn with the matrix matching mixed standard solution series. As shown by the results, linear relationships between the corresponding peak areas and mass concentrations of 11 neonicotinoid insecticides were kept in definite ranges, with detection limits (3S/N) in the range of 0.10-1.0 μg·kg^-1. Test for recovery was made by the standard addition method, giving results in the range of 60.3%-113%, with RSDs (n=6) of the determined values not more than 12%. This method was used to analyze 17 actual fish samples, and fipronil was detected in one yellow croaker sample, with the detection amount of 14.6 μg·kg^-1.