The invention discloses a preparation method of an anti-mildew agent for feeds, and belongs to the technical field of feed additives. The preparation method of the anti-mildew agent for feeds comprises the following steps of ball-milling vermiculite, water and betaine so as to obtain a ball-milled material; performing ultrasonic oscillation on the ball-milled material, a hydrochloric acid solutionand cysteine sulfonate, and carrying out freeze-drying so as to obtain a dried material; carrying out fermentation by using distilled water, the dried material, glucose, a yeast extract, edible maggot, fungus powder, disodium hydrogen phosphate and sodium dihydrogen phosphate, and carrying out drying, crushing and sieving so as to obtain sieved granules; putting acetone, ethanol, the sieved granules, diacylglycerol and an additive into a reactor, discharging gas by using trimethylamine, adding an catalyst, carrying out heated stirring, carrying out cooling to room temperature, and carrying out standing; after standing, discharging materials, carrying out filtration, conducting washing by using petroleum ether, diethyl ether, acetone and ethanol, and carrying out drying so as to obtain a dried material; grinding the dried material, a nutrient supplement agent and sodium humate, and collecting the ground material so as obtain the anti-mildew agent for feeds. The anti-mildew agent for feeds solves the problems of the existing anti-mildew agents for feeds, namely liable causing of anorexia and refusal to eat, and slow growth caused by reduced feed intake caused by abdominal fullness of livestock after eating the existing anti-mildew agents for feeds.