The invention relates to the sodium dehydroacetate dihydrate and a preparation method thereof. The sodium dehydroacetate dihydrate per mole comprises 2 moles of water. The preparation method comprises the following steps: dissolving the sodium dehydroacetate as a raw material in water at 60-80 DEG C to obtain a solution, wherein the mass concentration of the solution is 25% to 33%; after the dehydroacetate dehydrate is completely dissolved, decolorizing and filtering the solution with active carbon; under stirring, carrying out cooling crystallization at a cooling speed 15-20 DEG C/h to reduce the temperature to 5-15 DEG C, and growing crystals for 20-40 minutes; and filtering crystal slurry, washing, and drying to obtain the sodium dehydroacetate dihydrate product. The preparation method disclosed by the invention is simple in operating conditions and shorter in preparation time, the purity of the product is about 99%, mass yields of raw materials and the product are about 50% by dry basis. The product has high crystallinity, smooth crystal surface and uniform size. The sodium dehydroacetate dihydrate can be used as an active ingredient of mildew and corrosion resistant preservative of food or feed.