Methods for preparing black soybean hull extracts and uses thereof. The methods can comprise: (1) extracting crushed black soybean hull with aqueous solution containing enzyme and filtering the extracted liquid; (2) ultrafiltration of the extract liquid with an ultrafiltration membrane; (3) absorbing the ultrafiltration liquid with an absorption resin or an ion exchange resin, and elution with ethanol to obtain purified extract liquid; (4) concentrating the purified extract liquid under vacuum; (5) spray drying the concentrated liquid. The extract can contain: 10-45% of cyanidin, 10-25% of catechin, and 40-80% of OPC, and with a total polyphenol content as high as 70-100%. The extract can be used for preparing drugs or food containing antioxidants, or used for improving metabolic syndrome or eyesight.