The invention discloses a preparation method of a 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, haws, radix angelicae sinensis, momordica cochinchinensis, 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, 10-20 parts of radix bupleuri, radix scutellariae, 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, and uniformly mixing the walnut green seedcase granules with the Chinese yam starch paste, the fermented glycine max and the mixed granules 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 until the water content is not more than 5% so as to obtain the feed additive.