Abstract: A method for the simultaneous determination of fipronil and its metabolites(fipronildesulfiny,fipronilsulfone and fipronil sulfide)residues in milk was developed by ultra high performance liquid chromatography-tandem mass spectrometry(UPLC-MS/MS). The milk sample was extracted and salted out,and the Oasis PRiME HLB solid phase extraction column was purified by pass-through purification. The acetonitrile-water was used as mobile phase,and the HSS T3 column(2.1 mm×100 mm,1.8 μm)was chromatographed in electrospray negative ion mode. Then,the multiple reaction monitoring (MRM) method was used for measurement,and external standard method was used for quantification.The results showed that,the fiproniland its metabolites showed a good linear in the range from 0.05 to 5.0 μg/L,and the correlation coefficients r≥0.9976. The limits of detection(LODs)were 0.01 μg/kg and the limits of quantification(LOQs)were 0.05 μg/kg. At spiked levels of 0.5,1,10 μg/kg,the recoveries of the four target analytes were between 80.6% and 101%. The relative standard deviations were between 1.0% and 6.1%. The method was simple and sensitive,and couldbe suitable for the analysis of fipronil and its metabolites residues in milk.