The present invention discloses a fattening agent for regulating development levels and promoting muscle growths of cattle and sheep and a preparation method thereof. The fattening agent can regulate the development levels of the cattle and sheep, change the growth characteristics, promote the muscle growth, improve the immunity and feed digestibility, shorten the fattening cycle, can very play the fattening functions in the early, mid and late stages of the fattening period, reduce the costs of feeding, and improve the breeding efficiency. The fattening agent can rapidly decompose the antibiotic resistance within the cattle and sheep, and greatly enhance the resistance. The cattle and sheep do not need to take medicines or injections, and the fattening agent can replace health-care drugs and other antibiotics in the feed, and can also replace the various additives of probiotics, mildew preventives, compound enzyme preparations, etc. The breeding houses are free of bad smells. The fattening agent reduces respiratory diseases, saves medicines, efforts and money, improves efficiency, saves the feed for about 10%, is strong in attractant, enables the animals to love eat and grow fast, can break down crude proteins, crude fibers, pectin, etc. in the feed, which are not easy to digest and absorb, and unprecedentedly increases the digestion and absorption rate.