The invention relates to a method for preparing wheat bran dietary fiber by utilizing ultrasound-assisted enzymatic hydrolysis, and in particularly belongs to the technical field of the agricultural and sideline product development, the functional food additive and the nutritional heath caring. The invention is characterized in that the invention adopts wheat bran as the raw material, and the wheat bran dietary fiber preparation is realized through utilizing the ultrasound treatment, and removing starch and protein by enzymatic method, and then the preparation of fine dietary fiber with different grain size is realized by crumbling with a grinder. The invention realizes the effective value increment and the utilization of the wheat bran, the prepared functional dietary fiber has important physiological function, and thereby can realize the functions of preventing constipation, colon cancer, intestina diverticulum, hemorrhoid and varicose veins of the lower extremities; the serum cholesterol is lowered, and the heart disease caused by coronary sclerosis can be prevented; the sensitivity of the peripheral nerve tissue to the insulin is improved, and the blood sugar level of the diabetes patient can be regulated, the invention is very useful for human health, and the economic benefit as well as the social benefit are greatly increased.