The invention discloses a method for extracting a deoxynojirimycin monomer. The method comprises the following steps of: soaking a mulberry raw material in acidic aqueous solution, and extracting for 1 to 3 times; adsorbing extracting solution on a cation exchange resin, washing, and eluting by using ammonia water; performing reverse osmosis pre-concentration on eluent, recovering ammonia, dissolving an intermediate in a hydrophilic solvent, performing catalytic conversion, allowing the solution to pass through a gel chromatographic column, and eluting the hydrophilic solvent; recovering the solvent, dissolving in a mixed solvent, allowing the mixed solvent to pass through the chromatographic column, and eluting the mixed solvent; and recovering the mixed solution to obtain pure deoxynojirimycin products. The method is low in energy consumption and environment-friendly, and the pure deoxynojirimycin products with various purity grades of 10 percent, 50 percent, 99 percent and the like.