The invention discloses a granulating-dedicated coarse feed composite mildew inhibitor and a preparation method thereof, especially relates to a dedicated composite mildew inhibitor for making granulated coarse feed and a preparation method thereof, and aims to solve the problem of no a mildew inhibitor product, which is dedicated for coarse feed in the feed market at present. The formula of the composite mildew inhibitor comprises following components: calcium propionate, potassium sorbate, sodium benzoate, dimethyl fumarate, zeolite powder, aflatoxin degrading enzyme, and carriers. The preparation method comprises following steps: first, weighing 4.5 kg of calcium propionate, 5 kg of potassium sorbate, 4.5 kg of sodium benzoate, 6.0 kg of dimethyl fumarate, and 8.0 kg of aflatoxin degrading enzyme; secondly, pre-mixing the components in a small feed mixing machine for 10 minutes, then adding 27.0 kg of zeolite powder and 45.0 kg of corncob powder, putting the mixture into the small feed mixing machine again, and mixing for another 15 minutes so as to obtain the product. The granulating-dedicated coarse feed composite mildew inhibitor has the advantages of no toxicity, no accumulation in animal bodies, and no affection on nutrition value and palatability of coarse feed.