The invention relates to a feed additive for increasing immunity of mutton sheep and a preparation method thereof. The feed additive disclosed by the invention is characterized by being prepared from grass carp powder, alfalfa, calcium hydrophosphate, fish meal, mountain flour, aminophylline, choline chloride, lysine, methionine, complex vitamins, copper sulphate, ferrous sulphate, zinc sulphate, potassium iodide, sodium selenite, cobalt chloride, L-lactic acid, small peptide protein, casein, complex carbohydrate, oligosaccharides, bacillus licheniformis, bacillus subtilis, subprostrate sophora, chrysanthemum, pseudo-ginseng, eclipta alba, astragalus membranaceus, glossy privet fruit, almonds, dried tangerine or orange peel and bistort rhizome. The preparation method disclosed by the invention comprises the following steps: dissolving small peptide protein, casein, complex carbohydrate, oligosaccharides, bacillus licheniformis and bacillus subtilis in water at 30 DEG C, crushing an alfalfa and Chinese herbal medicine mixture, sieving the crushed mixture through a 80-mesh sieve, mixing the various components, and drying at 70-75 DEG C to obtain the mutton sheep feed additive. The feed additive disclosed by the invention is rich in nutrient substances and free from adverse effects to health of goats and any side effect, and is capable of increasing production performance, improving feed quality and promoting growth and breeding.