The invention discloses a preparation method of fermented corn straw feed for milk cows. The preparation method comprises the steps as follows: drying corn straw until the moisture content is 8%-12%,then, crushing the dried corn straw into sections 1-2 cm long, and stacking the sections for later use; and uniformly mixing the corn straw, traditional Chinese medicine dregs, a complex microbial agent, wheat bran, urea and water, and performing sealed fermentation, natural air-drying and crushing to obtain the feed. The traditional Chinese medicine dregs are wastes left after production of Chinese patent medicines and comprise, in parts by weight, 25-30 parts of radix astragali, 15-18 parts of radix paeoniae alba, 10-14 parts of radix bupleuri, 20-25 parts of radix codonopsis, 18-22 parts ofsemen vaccariae, 6-9 parts of herba leonuri, 10-14 parts of fructus lycii, 15-23 parts of fructus crataegi, 10-15 parts of malt and 8-14 parts of radix glycyrrhizae. The corn straw and waste traditional Chinese medicine dregs are used as main raw materials, waste resources are reasonably utilized, the economic and ecological benefits are very high, and the prepared feed for the milk cows also hasfunctions of increasing the milk yield of the milk cows and improving the milk quality.