The invention discloses a preparation method of a biological feed additive with nutrition and immunocompetence. The biological feed additive with nutrition and immunocompetence comprises the following components: 0.05 to 0.1% of aerobic bacteria, 0.05 to 0.1% of anaerobic bacteria, 0.3 to 0.5% of fermenting promoter, 5 to 15% of soybean meal, 5 to 10% of molasses, 15 to 25% of DDGS (Distillers Dried Grains with Soluble), 10 to 30% of cassava residue, 20 to 50% of bran meal, 5 to 15% of bran, 5 to 15% of maize cob powder, and 20 to 40% of maize slurry protein. The preparation method comprises the processes of activating bacteria, crushing raw materials, aerobically fermenting for 1 to 2 days, and then anaerobically fermenting for 3 to 7 days. The prepared biological feed additive brings piglet, medium and large pigs and sheep with high nutrition value, is high in palatability, and has effects of accelerating growth, improving intestinal health, controlling diarrhea, controlling constipation of sow, increasing birth weight and weaning weight of the piglet, improving the quality of eggshell and egg yolk, controlling feed-containing faeces, raising the lactation level of dairy cattle, as well as increasing butterfat and protein content.