The invention relates to the field of food ferment, in particular to seaweed ferment having the function of reducing blood sugar, and a preparation method of the seaweed ferment having the function ofreducing blood sugar. The seaweed ferment provided by the invention is prepared from raw materials of seaweed, ginseng, radix puerariae, Chinese wolfberry fruits, glossy ganoderma, bighead atractylodes rhizomes, radix ophiopogonis, panax notoginseng, rhizoma coptidis, radix astragali, corn stigma and the like. The preparation method comprises the following steps of after pretreatment of the raw materials, adding lactobacillus plantarum, lactobacillus rhamnosus and streptococcus thermophilus, and performing fermentation for the first time; adding chromium-enriched yeast and selenium-enriched yeast, and performing fermentation for the second time; performing distillation under reduced pressure so as to remove alcohol; performing compounding, adding fiber oligosaccharide, fructose and maltitol, adding chromium picolinate for reducing blood sugar and restraining blood lipid, and adding VC, VE and whey protein; and performing after-ripening so as to obtain the seaweed ferment having the function of reducing blood sugar finally. According to the seaweed ferment and the preparation method thereof disclosed by the invention, toxic and side effects of medicines are reduced, the function ofreducing blood sugar is strengthened, the fermentation efficiency is improved, the immunity of organisms is enhanced, the flavor is adjusted through an after-ripening compounding formula, the mouth feel is improved, and a ferment beverage which has the efficacy of reducing blood sugar and is excellent in mouth feel is formed.