The invention provides a feed additive specially used for laying hens in laying period and a preparation method thereof. The additive is prepared by mixing a Chinese medical herb extract preparation that protects the livestock and poultry from diarrhea, nutrients like methionine, lysine, vitamin E and auxiliary materials like medical stones and corn cob meal in proportion, and the parts by weigh is: 100 parts of Chinese medical herb extract preparation, 1 part of methionine, 0.5 part of lysine, 8 parts of vitamin E, 600 parts of medical stones and 290.5 parts of corn cob meal; the Chinese medical herb extract preparation is prepared in advance with raw medical materials of paulownia flower, purslane, water pepper, geranium wilfordii, sanguisorba, copperleaf herb and cactus in proportion by picking, washing, cutting, first aqueous extract, second aqueous extract, combining for decoction, concentrating, absorbing and parching. The weight proportion of the raw medicinal materials is: 29-35 parts of paulownia flower, 22-28 parts of purslane, 20-26 parts of water pepper, 17-23 parts of geranium wilfordii, 17-23 parts of sanguisorba, 13-17 parts of copperleaf herb and 8-12 parts of cactus. Silicon dioxide is chosen as sorbent in the production process and the addition of the silicon dioxide as one of the compositions of the Chinese medical herb extract preparation weighs 85 percent of the raw medicinal materials.