The invention discloses an animal feed additive and a preparation method thereof. The additive comprises 20 g of codonopsis pilosula, 20 g of astragali radix, 10 g of malt, 10 g of fructus crataegi(stir-fried), 250 g of bean pulp, 250 g of cottonseed meal, 250 g of rapeseed meal, 100 g of sweet pepper pulp and 90 g of chrysanthemum powder. The preparation method of the additive comprises the following steps: firstly, conducting primary fermentation on the codonopsis pilosula, the astragali radix, the malt and the fructus crataegi(stir-fried) through using lactobacillus bulgaricus, wherein liquid fermentation is adopted in the primary fermentation, and the bacterium-inoculating amount is 20%; secondly, conducting secondary fermentation on the product subjected to the primary fermentation with 250 g of bean pulp, 250 g of cottonseed meal, 250 g of rapeseed meal, 100 g of sweet pepper pulp and 90 g of chrysanthemum powder through using saccharomycetes, so that a finished additive is obtained. According to the invention, effective ingredients in the traditional Chinese herbal medicines can be fully released during the primary fermentation; cellulose, protein and carbohydrates in the traditional Chinese herbal medicines and enriched plant materials can be resolved and converted during the secondary fermentation, as a result, rich organic acid and amino acid are produced, the nutrient content is further enriched, and the mouthfeel of the animal feed is better.