The invention discloses a microbial feed additive capable of enhancing immunity of fish and an application of the feed additive. The microbial feed additive includes, by weight, 16-22 parts of fructus forsythiae, 16-19 parts of herba andrographitis, 12-17 parts of artemisia apiacea, 15-21 parts of Chinese rhubarb, 1-2 parts of angelica sinensis, 13-20 parts of houttuynia cordata, 10-15 parts of radix glycyrrhizae, 7-11 parts of acanthopanax roots, 0.6-0.9 parts of garlic, 2-3 parts of pericarpium citri reticulatae, 3-5 parts of eucommia ulmoides, 0.8-1.3 parts of bacillus subtilis, and 1-1.4 parts of lactobacillus plantarum. The microbial feed additive is free of toxic and side effects, has no pollution and residual, is free of generating drug resistance, can promote secretion of antibodies by a host, improves activity of immune cells of the host and improves immunity and resistance of body. The microbial feed additive also maintains microecological balance in intestinal tracts and best immune state, and also improves digestion and absorption rate of nutrients.