The invention discloses a preparation method of a sheep feed additive. The preparation method comprises the following steps: a step I, taking walnut green seedcases, crushing the taken walnut green seedcases to obtain walnut green seedcase granules, taking Chinese yam, making the taken Chinese yam into Chinese yam starch paste, taking glycine max, and making the taken glycine max into fermented glycine max; a step II, firstly taking radix astragali, medicinal evodia fruits, fructus forsythiae, figs, haws, radix angelicae sinensis, premna fulva craib, radix isatidis, common andrographis herbs, pollen containing bacillus thuringiensis, poria cocos, folium isatidis, licorice roots, fructus aurantii, pseudo-ginseng, lalang grass rhizomes, mulberry leaves, radix bupleuri, radix scutellariae, coffee beans, pinellia tubers, honeysuckle flowers, mint, fried millet sprouts, mulberry leaf powder, cassava residues, peanut meal, tea polyphenols, usnic acid, radix ophiopogonis and lotus seeds, then performing uniform mixing, performing crushing to obtain mixed granules, then taking the walnut green seedcase granules, the Chinese yam starch paste and the fermented glycine max, uniformly mixing the walnut green seedcase granules with the Chinese yam starch paste, the fermented glycine max and the mixed granules, and performing vacuum drying so as to obtain a primary feed additive; a step III, soaking the primary feed additive in tea soup for 24-48h; and a step IV, drying the primary feed additive so as to obtain the feed additive.