The present invention relates to Rhus verniciflua Stokes extract having an increased content of an active flavonoid compound through a gas bubbling treatment, a method for preparing the Rhus verniciflua Stokes extract, and a method for converting fustin into fisetin through the gas bubbling treatment of a solution containing fustin. The method for preparing the Rhus verniciflua Stokes extract of the present invention can convert the conventional Rhus verniciflua Stokes extract having high content of fustin and low content of fisetin into a Rhus verniciflua Stokes extract having high content of fisetin. Therefore, the Rhus verniciflua Stokes extract of the present invention can be commercialized into natural anticancer agents, anticancer and cancer prevention food products, and health functional foods due to the excellent bioactivity thereof, and promote the development of relevant industries through the high added-value of Rhus verniciflua Stokes. In addition, the method for converting fustin into fisetin can be applied to other natural substances and be used as a method for enhancing the functionality thereof by increasing the content of a high active flavonoid, thereby becoming an enabling technology that can lead high-functionalization of natural substances.