The invention discloses a natto preparing method for increasing the inhibition ratio of a natto water extract to alpha-glucosidase. The natto preparing method comprises the following steps: adding bacillus subtilis natto to cooked soybeans, and performing uniform mixing, wherein the mass volume ratio of the cooked soybeans to the bacillus subtilis natto is (30-60)g:1ml, and the OD<600> of the bacillus subtilis natto is 0.5-1.2; performing fermentation, wherein the fermentation temperature is 28-32 DEG C, the fermentation humidity is 60-80%, and the fermentation time is 55-72h; and performing after-ripening, wherein the after-ripening temperature is 2-6 DEG C, and the after-ripening time is 24-48h. According to the natto preparing method disclosed by the invention, bacterial liquid at a quick growth period is selected, and besides, the natto fermentation condition is improved, so that the inhibition ratio of the prepared natto to the alpha-glucosidase can be increased by 6 times or above.