The invention discloses a feed additive for preventing lactation-period-calf diarrhea and an application method thereof. According to the feed additive, natural astaxanthin, sodium humate, beta-hydroxyl-beta-methylbutyric acid, glutamine, xylooligosaccharide and a Chinese herbal medicine additive (agrimonia pilosa ledeb, honeysuckle, Chinese pulsatilla roots, hawthorns, pomegranate bark, talc andlicorice roots) are compounded, the balance of gastrointestinal flora of lactation-period calves is conditioned, development of gastrointestinal mucosa is promoted, the damage and influence of pathogens and unfavorable factors on the calves are reduced, digestion and absorption of nutritional ingredients are increased, the body immunity is improved, the suffered oxidation stress and oxidative damage are relieved, the anti-stress ability is improved, and therefore the aims that occurrence of lactation-period-calf diarrhea is avoided, absorption of feed nutrients is promoted, and the health level and production performance of the calves are improved are achieved.