Abstract: A method of ultra performance liquid chromatograghy-electrospray tandem mass spectrometry (UPLC-MS/MS) was studied for determination of β₂-agonist in feed. The sample was extracted by 0.1 mol/L hydrochloric acid-methanol mixed solvent, purified and enriched with cationic solid phase extraction column， and which was separated by liquid chromatography on C₁₈ column with mobile phase of 0.1% formic acid in methanol(gradient elution), then analyzed in UPLC-MS/MS with electrospray ion source(ESI) under multi-reaction monitor mode(MRM). The quantification was performed with external standard method. The method has a lower limit of quantitation 50 μg/kg. Spike recoveries of three concentration addition levels(50, 100, 200 μg/kg) ranged in 71％—103% with relative standard deviations in 5.7％—9.8％. The method is fast, accurate and sensitive. It is suitable for rapid qualitative and quantitative detection of β₂-agonist in animal feed.