The invention discloses a method for preparing functional xyio-oligosacchandes by taking agricultural wastes as raw materials and using ultrasonic treatment and bio-enzyme degradation. The method comprises the following steps: hydrolyzing the agricultural wastes after the ultrasonic treatment by using endo xylanase generated by Streptomyces rameus, converting hemicellulose in the agricultural wastes into the high-quality xyio-oligosacchandes, and preparing a xyio-oligosacchandes product having low xylose content and the polymerization degree between 2 and 5.