The invention relates to a method for preparing a feed for cattle, sheep and herbivorous animals, by fermentation of pennisetum giganteum. The feed comprises the following preparation raw materials in parts by weight: 50-65 parts of pennisetum giganteum, 5-8 parts of zymocyte, 6-10 parts of molasses, 10-15 parts of a grain feed, 0.4-1 part of urea phosphate, 0.5-0.7 part of table salt, 1-2 parts of trace elements, 0.7-1.0 part of mixed enzymes, 8-10 parts of papaya peel, and 7-12 parts of pineapple peel. The pennisetum giganteum is used for fermenting for producing the feed for the cattle, the sheep and the herbivorous animals, and belongs to a forage reusing project. Waste residues are not discharged, and the feed is environment-friendly. Through fermentation, the feed is effectively decomposed, and converted into micromolecule substances, such as glucose and amino acids, which are easy to digest and absorb by animals, so that the nutrient value and the utilization conversion rate of the feed are increased; and besides, in the fermentation process, harmful infectious microbes and pathogenic microorganisms, attached to the feed are effectively restrained, and the feed quality is improved. After fermentation, the feed is rich in nutrition and good in palatability.