A high performance liquid chromatography-electrospray ionization mass spectrometry method was developed for the determination of aconite alkaloids. It was used to investigate the degradation of alkaloids of Radix Aconiti Lateralis Preparata during decoction. Six alkaline degradation products were identified, and the degradation regularity of diester-diterpenoid alkaloids was confirmed during the test using standards. The dynamic changes of the amount of aconite alkaloids in the decoction of Radix Aconiti Lateralis Preparata were supervised. Along with the increase of decoction time, the concentrations of diester-diterpenoid alkaloids and lipo-alkaloid decreased significantly. The results can provide a scientific basis for the safety use of aconite.