The invention relates to a feed additive for preventing avian influenza; the feed additive is composed of the following raw materials in parts by weight: 30-60 parts of folium isatidis, 20-40 parts of common St.John's wort herb, 10-30 parts of radix astragali, and 10-25 parts of liquorice. The invention also discloses a preparation method of the feed additive. The feed additive not only can prevent and control avian influenza but also can improve the immunity of animals, and has the characteristics of small side effects, low cost, simple process and easy industrialization.