A method for separating and purifying dehydroandrographolide integrates the methods of solvent extraction, column chromatography and supercritical fluid anti-solvent crystallization to process andrographis paniculata extract to prepare high-purity dehydroandrographolide. Chloroform is first used for extracting the extract. Fillers for column chromatography (stationary phase) adopt silica gel and the like. The mobile phase is the mixed solvent of petroleum ether and ethyl acetate. The supercritical fluid CO2 anti-solvent crystallization and purification are carried out under the conditions that the pressure is less than or equal to 15MPa and the temperature is less than or equal to 65 DEG C. The purity of the product is higher than 99%. The separating and purifying process features easy and simple method and low cost and is easy to be applied to industrial production.