The invention provides a feed additive for optimizing an animal intestinal tract microbial population. The feed additive is characterized in that the feed additive comprises lactobacillus casei, isomaltooligosaccharide, butyric acid bacteria and carriers, the number of the lactobacillus casei in each feed additive is 100-200 billions, the weight of the isomaltooligosaccharide in each feed additive is 400-600g, the number of the butyric acid bacteria in each feed additive is 50-100 billions, and the weight of the carriers in each feed additive is 200-300g. According to the feed additive, the lactobacillus casei, the isomaltooligosaccharide, the butyric acid bacteria and the carriers are synergistically matched, the animal intestinal tract microbial population is adjusted, micro-ecological balance in animal organisms can be facilitated, definite health effects can be generated, so that host micro-ecological balance is improved, animal immunities are remarkably improved, composition components of the feed additive can be dissolved into water, the feed additive can be added into daily ration or drinking water of livestock and poultry, is simple and convenient to use and has the advantages of safety, high efficiency, specificity, less additive amount and the like, culture benefits are improved, economic benefits and social benefits of animal husbandry are uniformly coordinated, and sustainable development of the animal husbandry is achieved.