A method for preparing dehydrocavidine, dehydroapocavidine and their respective composition is provided. The composition is first prepared by isolating and purifying the quaternary ammonium alkaloid components from the medicinal plant “Yan Huang Lian” (Corydalis saxicola Bunting) through the processes of solvent extraction, water-phase organic extraction, crystallization and recrystallization, and then drying to obtain said composition containing dehydrocavidine and dehydroapocavine. When necessary, the composition or their crude extracts can be separated by chromatography to obtain dehydrocavidine or dehydroapocavidine. Dehydrocavidine, dehydroapocavidine or their respective composition can be used in manufacturing medicines for treating viral hepatitis, hepatic injury, influenza, AIDS, tumors or arrhythmia.