TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The method comprises extracting clove bud with hydrophilic organic solvent, concentrating the extract, extracting oil with petroleum ether and ethyl acetate, respectively, concentrating and drying to obtain crude extract; purifying the crude extract by silica gel column chromatography, where petroleum ether/ethyl acetate gradient mixing system in the volume ratio of 2:1-0:100 is used as mobile phase, then collecting the eluate, concentrating to dryness, and re-crystallizing with ethyl acetate or ethanol, to obtain the product with purity of greater than or equal to 98%. The purification of crude extract by silica gel column chromatography comprises taking petroleum ether/ethyl acetate as mobile phase with gradient elution volume ratio of 2:1, 1:1, 1:2 and 0:100, and collecting the eluted fraction of petroleum ether/ethyl acetate with volume ratio of 1:2 and 0:100. The preparation method comprises crushing dried clove buds into coarse powder, heating to reflux extract with 80-90 volume% methanol or ethanol three times, each time for 0.5-2 hours, combining the extracts, vacuum concentrating to obtain the extract, degreasing with 4-6 volumes of petroleum ether as extraction solvent 3 times, centrifuging, extracting with 8-15 times volume of ethyl acetate three times, vacuum concentrating to obtain pale yellow powder of (2 alpha ,3 beta )2,3-dihydroxy-olean-12-en-28-oic acid crude extract; purifying the crude extract by silica gel column chromatography, where petroleum ether/ethyl acetate gradient mixing system in the volume ratio of 2:1, 1:1, 1:2, 0:100 is used as mobile phase, collecting the eluted fraction of petroleum ether/ethyl acetate with volume ratio of 1:2 and 0:100, analyzing same composition eluate by thin layer chromatography, combining, vacuum concentrating to dryness to give a white powder; saturating the white powder with hot solution of ethanol or ethyl acetate, rapidly cooling the solution in an ice bath, completely filtering to remove impurity, and re-crystallizing the filtrate. Preferred Components: The hydrophilic organic solvent is 80-100 volume% acetone, methanol or ethanol.