A method for preparing 2,2′,4,4′-Tetrahydroxy-3-(2″-hydroxy-3″-methylbutyl-3″-alkenyl)chalcone includes the following steps: subjecting a Morus alba leaf to extraction with an aqueous solution of methanol or ethanol having a volume fraction of 40%-100%, concentrating an extract to remove methanol or ethanol and dissolving in water, subjecting to extraction with petroleum ether and ethyl acetate successively, and concentrating an ethyl acetate extract to obtain a paste; chromatographing the paste over a silica gel column using chloroform-methanol, collecting an eluate where the volume ratio of chloroform-methanol is 95/5; chromatographing the eluate over a reversed-phase column using methanol-water, collecting an eluate where the volume ratio of methanol-water is 60/40, and thereby the compound is obtained.