The invention discloses a feed additive for preventing diarrhea in lactating calves and an application method thereof. The present invention uses natural astaxanthin, sodium humate, β-hydroxy-β-methylbutyric acid, glutamine, xylooligosaccharides and Chinese herbal medicine additives (agrimony, honeysuckle, pulsatilla, hawthorn, pomegranate peel, talc, licorice ) to formulate formulas to regulate the balance of gastrointestinal flora in lactating calves, promote the development of gastrointestinal mucosa, reduce the damage caused by pathogens and unfavorable factors, increase the digestion and absorption of nutrients, improve the body's immunity, and relieve the oxidative stress suffered Stress and oxidative damage, improve the anti-stress ability, so as to prevent the occurrence of diarrhea in lactating calves, promote the absorption of feed nutrients, and improve the health level and production performance of calves.