Abstract: The DDA were studied by ESI-MSn in decocting solutionof Radix Aconiti Lateralis Preparata (RALP) with Bulbus Fritillariae Cirrhosae(BFC), Bulbus fritillariae Thunbergii(BFT), respectively. Comparing with the ratio of relative abundant of DDA with and internal standard, it was found that the DDA were higher in decocting solution of RALP-BFT than that of RALP, while the DDA in decocting solution of RALP-BFC were lower than that of RALP. We deduced that the BFT inhibited hydrolysis of diester-diterpenoid alkaloids in decocting process with RALP, leading to the high toxicity of the RALP-BFT decoction.