Abstract: Mequindox and its possible metabolites in body wall of Apostichopusjaponicus were identified by high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Acetonitrile extraction and hydrochloric acid hydrolysis-SPE extraction were conducted to extract different kinds of metabolites. Metabolites were separated on an Acquity^TM UPLC^® BEH C18（2.1 mm×50 mm×1.7 μm）column by gradient elution with acetonitrile-water (containing 0.1% formic acid) as mobile phase, and detected by positive ions electrospray ionization (ESI) mass spectrometry with fullscan-data dependent scan. Through comparison of MS¹ and MS² ions with the reference standard, 3-methyl-quinoxaline-2-carbosylic acid (MQCA) is identified as a metabolite of mequindox in Apostichopusjaponicus. The other identified metabolites include D2, D3, D4, and which the fragmentation pathways are deduced.