A method was developed for the determination of azithromycin in formula feed for fattening pigs by liquid chromatography-tandem mass spectrometry（HPLC-MS/MS）.The sample was extracted with acetonitrile under ultrasonic assistance.Oasis MCX solid phase extraction column was used for purification，and the chromatographic separation was performed on a Hypersil BDS C18 column（2.4 μm，100 mm×2.1 mm） with gradient elution using 0.05 mol/L ammonium acetate-methanol（1∶9） as mobile phase.The detection of azithromycin was performed with positive ion under multiple reaction monitoring（MRM） mode.The internal standard isotope dilution method was used for quantification.A good linearity（r2=0.999 1） was obtained for azithromycin in the range of 1-250 μg/L.The limit of detection（LOD，S/N≥3） and the limit of quantitation（LOQ，S/N≥10） of azithromycin in the formula feed were 2.8 μg/kg and 8.4 μg/kg，respectively.Average recoveries ranged from 87.0% to 106.6% for the formula feed at spike levels of 0.5，1，5，10 mg/kg.The intra-RSDs for azithromycin were 0.58%-5.4% and the inter-RSDs were 3.1%-5.4%.The method is accurate and sensitive，and the matrix interference is small.The method could be applied in the determination of azithromycin in formula feed.