The invention relates to a preparation method for Japanese St. Johnswort extract, and characterized by the following steps: taking Japanese St. Johnswort, the medicinal materials, boiling 1-3 times, each time adding water 5-30 times of said materials, each time boiling for 10min.2 hr., filtering, combining water boiling solution; filtering by ceramic membrane with aperture of 0.1 mum-0.8 mum , again filtering by organic membrane with molecular cut off of 2000-8000, filtrate being concentrated into relative density as 1.05-1.20,laying aside to have sediment, filtering, taking sediment to be dried , thus obtaining said Japanese St. Johnswort extract , the general flavone amount making up more than 50% in said extract. |