［［Objective ］］ To establish a method for simultaneous determination of chlorothalonil and its metabolite 4-hydroxychlorothalonil residues in animal -derived food by ultra -high performance liquid chromatography -tandem mass spectrometry.［［Methods ］］ The samples were extracted by oscillating with acetonitrile solution of 1% acetic acid,defatted with n -hexane,purified by matrix dispersible solid phase extractant (PSA,anhydrous magnesium sulfate ),separated by Waters BEH C18 column under a gradient elution program using 0.1% formic acid aqueous solution and acetonitrile as the mobile phases.The anion multiple reaction monitoring model (MRM ) of atmospheric pressure chemical ionization source (APCI ) was used for the determination,and the matrix matching external standard method was used for quantitative determination.［［Results ］］ The linear relationships of chlorothalonil and 4-hydroxychlorothalonil were good in the range of 5~500 μg/L,and the limits of quantitation were 0.005 mg/kg.The average recoveries of chlorothalonil and 4-hydroxychlorothalonil were 69.3%~111.0% and 65.3%~117.9%,respectively.The relative standard deviations of chlorothalonil and 4-hydroxychlorothalonil were 1.6%~10.8% and 1.1%~9.0%,respectively.［［Conclusion ］］ The method is simple,rapid,sensitive,accurate and reliable for simultaneous determination of chlorothalonil and its metabolite 4-hydroxychlorothalonil in animal -derived food.