A sensitive high performance liquid chromatography tandem mass spectrometric（LC-MS/MS) method was developed for the determination of perchlorate in milk.The perchlorate residue in milk was extracted with 1% acetic acid-acetonitrile（1∶4,by volume) and the protein of milk was precipitated by acetonitrile.The extract was centriged at 6 000 r/min for 20 min，and the supernatant was cleaned up with 0.2 μm nylon filter,On-Guard Ⅱ RP column,On-Guard Ⅱ Ag column and On-Guard Ⅱ Ba column in turns.The separation of perchlorate was carried out on a C12（Synergi 4u MAX-RP80A) by gradient elution using 1% formic acid（A)-methanol（B)as mobile phase with flow rate of 0.3 mL/min.The perchlorate was analyzed by tandem mass spectrometry with negative ion electrospray under multiple reaction monitoring（MRM) mode by selecting m/z 98.9/82.8 as qualitative ions．The extract was quantified by the external standard method.The result indicated that a good linearity was obtained for perchlorate concentration in the range of 0.1-1 000 μg/L.The average recoveries of perchlorate from 3 kinds of milk in market at three concentration levels of 1.0,5.0,10 μg/L ranged from 84% to 94% with RSDs of 1.2%-1.7%.The quantitation limit（LOQ) and detection limit（LOD) of perchlorate were 0.1 μg/L and 0.03 μg/L,respectively.The method is suitable for the determination and confirmation of perchlorate in milk with good repeatability and high sensitivity.