Abstract: The sextuple mass spectrum（MS¹-MS⁶）of Monacolin J was studied in positive mode by electrospray ionization trap mass spectrometry. It can be inferred that most fragments are formed by loss of H₂O, CH₃COOH or polyene hydrocarbon ions C_nH_2n（n=1-5）. Some of smaller fragments (molecular weight < 200) are obtained by hydrogen-immigration and Retro Diels-Alder reaction, which present the mass spectrum split characterization of alkenes and cycloolefin. These mass spectrum rules can provide reference date for the structure-function relationship study of Monacolin J and other statins.