To enhance the physicochemical and functional properties of insoluble dietary fiber (IDF) from buckwheat straw, we investigated the effects of alkaline hydrogen peroxide (AHP) treatment. Electron microscopy showed that the IDF had regular and compact tubes that turned into wrinkled lamellar products. After AHP treatment, X-ray diffraction indicated that the crystalline structure of the IDF was perturbed. And an undesirable decrease was observed in the content of hydroxybenzoic acid derivatives, hydroxycinnamic acid derivatives, flavonoids and the antioxidant capacity of IDF modified by AHP; however, the hydration properties (such as water holding capacity), α-amylase inhibition activity and glucose adsorption capacity of IDF were significantly enhanced by AHP. Furthermore, AHP led to a redistribution of monosaccharides in soluble dietary fiber and IDF, an interesting finding hinting at the mechanism and potential applications of AHP modification of IDF. In this study, AHP enhanced the physiological and functional properties of buckwheat straw IDF.