A sensitive high performance liquid chromatography-tandem mass spectrometry(LC/MS/MS) method was developed and validated for the determination of an azo dye p-2(-hydroxy-l-naphthylazo) benzene sulfonic acid sodium salt(Acid orange Ⅱ)in Carthamus tinctorius L..After having been extracted from Carthamus tinctorius L.samples by 60% methanol,Acid orange Ⅱ and internal standard,direct red 28 were separated on a C18 column with a gradient mode using a mixture of methanol and 5 mmoL · L-1 NH4Ac as a mobile phase.Detection was performed on Thermo Fisher TSQ Quantum Accela mass spectrometer equipped with an ESI source operated in the negative-ionization mode.Quantitation of Acid orange Ⅱ was accomplished by computing the peak area ratio(Acid orange Ⅱ fragment m/z 156/direct red 28 fragment m/z 152) and comparing them with calibration curve(r2 =0.9998).The method was in-house validated in terms of detection limits(LOD),quantitation limits(LOQ),linearity,sensitivity,accuracy and recovery.Good results in the low ng· g-1 level were obtained for LOD and LOQ.Calibration curves are linear in the 10.0-5000.0 ng · g-1 (ppb) range for Acid orange Ⅱ.