The invention discloses a method for preparing cubilose extract used in cosmetics. Steps are as follows: (1) cryogenic grinding cubilose, sieving, adding water 20 times the weight of the dry cubilose, ultrasonic extracting at room temperature for 1h, filtering, (2) adding water 30 times the weight of the dry cubilose into the filter cake, boiling for 60 minutes under mechanical stirring, (3) cooling to room temperature, regualting pH to 9.8 with Na2CO3 solution, adding alkali protease 1.5% the weight of the dry cubilose, carrying out hydrolysis in water bath at 55deg.C under mechanical stirring, (4) deactivating enzyme at 85deg.C for 20min, (5) cooling to room temperature, regulating pH to neutrality with thin HCl, (6) filtering, combining the filtrate with ultrasonic extract, and freeze drying to obtain powdered cubilose extract. The invention has simple process, good extract water-solubility, high extraction yield, rational product molecular weight distribution, good wetness protection, high product whitening activity, and good product stability; and avoid the influence of boiling to the thermo instable active ingredient in hydrolysis technology. The obtained cubilose extract is suitable for using as cosmetic compounding.