The invention discloses a low glycemic index sweet biscuit and a preparation method thereof, and relates to the technical field of food processing. The biscuit is prepared from the following raw materials in parts by mass: mixed coarse cereal powder, whey protein powder, white kidney bean powder, egg white, xylitol, erythritol, galactooligosaccharide, phytostanol ester, resistant dextrin and a bulking agent, the mixed coarse cereal powder is prepared by crushing quinoa, red beans, peas, oats, buckwheat, barley, red rice, coix seeds, millet and germinated brown rice. The mixed coarse cereal powder is adopted to replace wheat flour, so that the defect that the wheat flour is indeed diverse in vitamins and minerals is overcome; meanwhile, the glycemic index of the mixed coarse cereal powder is lower than that of wheat flour, the digestion speed is low, and the defect that blood sugar rises fast can be relieved; sucrose is replaced by erythritol and galactooligosaccharide, so that the calorie of the biscuit is reduced, and the biscuit is suitable for diabetics and people who lose weight to eat; egg white and xylitol are added; the intake taste of the biscuit is improved.