Objective This study aimed to investigate the functional characteristics of polyphenols from black soybean hulls (a by-product of black soybean) and their application in bread.Methods The polyphenol extracts were obtained from black soybean hulls via ultrasonic-assisted solvent extraction, followed by the isolation of free and bound polyphenols. The regulatory effect of these polyphenols on the starch digestion characteristics of bread was further analyzed.Results Free and bound polyphenols were isolated from polyphenols from black soybean hulls, with contents of 20.42 mg GAE/g and 9.47 mg GAE/g, respectively. The ABTS and DPPH radical scavenging rates of free polyphenols reached 88.85% and 80.04%, respectively. The addition of 0.25% polyphenol extracts decreased the rapidly digestible starch content in bread crumb and crust from 43.75% and 40.23% to 31.25% and 27.93%, respectively, while the resistant starch content increased to 45.32% and 50.59%, respectively. Molecular docking analysis revealed that components such as chlorogenic acid in polyphenols from black soybean hulls exerted competitive and noncompetitive mixed inhibition on α-glucosidase through hydrogen bonding and hydrophobic interactions.Conclusion Polyphenols from black soybean hulls can retard bread digestion by inhibiting α-glucosidase activity and modifying starch structure.