The invention provides a preparation method and application of Sapidolide A. Fruits of baccaurea plants are used as raw materials, are extracted by using a medium-low polar organic solvent and then are subjected to high-sample-loading-amount silica gel adsorption chromatograph separation and recrystalization to obtain a pure product of the Sapidolide A. By using the process, the yield of the pureproduct prepared from the raw materials is as high as 0.10 percent or above. Pharmacological experimental model screening proves that the Sapidolide A has the effects of protecting liver and promotingM2 type polarization of macrophage, and can be used for preparing a product which can be used for protecting the liver and adjusting immunity. According to the preparation method disclosed by the invention, novel application, including liver protection and immunity adjustment, of the Sapidolide A in the baccaurea plants is developed, the medicine action range of the baccaurea plants is expanded,and basis is provided for promotion, utilization and deep processing of traditional Chinese medicinal materials.