Abstract: A gas chromatography-tandem mass spectrometry(GC-MS/MS) method was established for the simultaneous determination of acetochlor, metolachlor and triadimefon residue in corn flour. The samples were extracted with acetonitrile for 3 h, and the upper acetonitrile layer was filtered with 0.22μm nylon membrane.The samples were then determined by GC-MS/MS. The results showed that the calibration curve was linear in the range of 0.005-1.0 mg/L. The determination coefficients(R2) was higher than 0.999. When the spiked levels were 0.025, 0.050 and 0.25 mg/kg, the average recoveries of the three pesticides were between 95%-101%, with relative standard deviation(RSD) between 0.7% and 2.2%(n=6). The limits of quantitation(LOQ) for the three pesticides were 0.05, 0.05 and 0.2μg/kg, respectively. This new method is accurate, sensitive, simple and suitable for the simultaneous determination of the above mentioned three pesticide residues in corn flour.