The invention discloses a method for separating and purifying 4,8-DHT from pecan exocarp and an application of 4,8-DHT. The method comprises the following steps: performing ultrasonic extraction to obtain a pecan exocarp extracting liquid by virtue of independent excitation type ultrasonic sounding, performing reduced pressure concentration, then passing through X-5 macroporous resin, performing gradient elution by using a mixed solution of water and ethanol, collecting an eluate obtained by eluting with a mixed liquid of water and ethanol in a volume ratio of 4 to 6, performing reduced pressure concentration to obtain a concentrated solution, performing silica-gel column chromatography, performing gradient elution by using a mixed solvent of petroleum ether and ethyl acetate, totally collecting 36 bottles of flow components by collecting a bottle of the flow components per 1 / 2 column volume, combining the tenth to eighteenth bottles of the flow components, performing reduced pressure concentration, and performing ethyl acetate crystallization to obtain a 4,8-dyhydroxy-1-tetralone crystalline compound. 4,8-DHT disclosed by the invention is of a 1:1 raceme, is less in impurity, has the purity of 99.5-99.8%, and ensures that the extraction rate reaches more than 94%; and the 4,8-DHT raceme has the effects on inhibiting seed germination and seedling growth of plants.