The invention discloses a low-sugar nutritional biscuit and a preparation method thereof. The low-sugar nutritional biscuit comprises the following components in parts by weight: 20-35 parts of dietary fiber powder, 20-25 parts of weak-gluten wheat flour, 10-15 parts of whole potato powder, 10-20 parts of corn flour, 10-25 parts of mung bean flour, 10-20 parts of butter, 8-12 parts of xylitol, 6-10 parts of eggs, 10-15 parts of stevia powder, 8-12 parts of peppermint powder, 3-8 parts of whey protein powder, 1-3 parts of cod liver oil and 3-5 parts of flaxseeds. The biscuit of the invention islow in sugar, high in nutritional value and good in taste and flavor. The preparation method of the invention enables the biscuit to be crisper, and the flavor of the biscuit to be more durable.