Abstract: The ESI-MS analytical method was established for studying the alkaloids content in the decoctions of Radix aconiti combined with Bulbus Fritillariae Thunbergii or Bulbus Fritillariae Cirrhosae. For the co-decoctions of Radix aconiti combined with Thunberg Fritillary Bulb contains more diester-alkaloids than that of co-decoctions of Radix aconiti combined with Bulbus Fritillariae Cirrhosae. When Radix aconiti was combined with Thunberg Fritillary Bulb, diester-alkaloids were mainly dissolved in water. But diester-alkaloids were mainly converted into lipo-alkaloids when Radix aconiti was combined with Bulbus Fritillariae Cirrhosae.