Abstract: Aim To study the change of chemical compound after decocting the flowers of Aconitum kusnzoffii Reichb (FAK) by analyzing the alkaloids in FAK and the decoction of FAK qualitatively. Methods The alkaloid extracts of FAK and alkaloid mixtures in the decoction of FAK were directly analyzed by electrospray ionization trap tandem mass spectrometry via by pump. Results Three novel aconitum alkaloids were found in FAK. In addition, it is found that both diester diterpenoid alkaloids and triester diterpenoid alkaloids were hydrolyzed in the decoction of FAK. The hydrolysis products of the diester-alkaloids are benzoylaconines and aconines; the hydrolysis products of the triester-alkaloids are 3-acetyl-aconines. Conclusion ESI-MSn method is suitable for the analysis of Aconitum alkaloids in the decoction of FAK as well as other Aconitum plants, which offers the advantages of convenience, speed, sensitivity and specificity.