The invention discloses a process for preparing gardenin comprising the steps of, A. airing cape jasmine dried fruits, disintegrating and sieving, B. charging petroleum benzene, supersonic extracting, and combining the filter liquor, reclaiming petroleum benzene to obtain vegetable oil, C. ethanol supersonic extracting, filtering, reclaiming ethanol, obtaining ethanol extract, D. eluting the ethanol extract with deionized water, E. depigmenting with ethanol, reclaiming the ethanol to obtain crude gardenin, F. removing impurities with 5% ethanol elution, E. depigmenting with 10% ethanol, concentrating the ethanol, drying and dewatering, thus obtaining gardenin.