A high performance liquid chromatography-tandem mass spectrometric method with on-line SPE(solid phase extraction) was established for the determination of 5 quinoxaline residues in feed.2 g feed was extracted with 10 mL 0.1% HCl-CH3OH(1∶1).The extract was 10 times diluted with 0。2% formic acid.The dissolution was enriched and purified with a reverse phase on-line SPE column on a double ternary liquid chromatograph.Target compounds were eluted with 0.2% formic acid and acetonitrile by gradient elution,and separated with an octadecyl silica bonded column.Target compounds were then determined by tandem mass spectrometry.The results showed that the linearities of 5 quinoxalines were good in the range of 50-25 000 μg/kg(r＞0.999).The LOD of the method was 25 μg/kg,and LOQ was 50 μg/kg.Recoveries of 5 quinoxalines were in the range of 72.6%-84.6%,while RSDs of both in the batch and between the batch were all lower than 10%.The method was proved to be more convenient,economical and stable than the traditional column SPE method.