The invention relates to a feed additive for increasing intestinal amino acid content of growing and fattening pigs and promoting protein synthesis as well as a preparation method and application of the feed additive, and belongs to the field of feeds. The feed additive comprises metalloprotease, fish meal, enzymolysis protein peptide, transferred glucuronidase genetic lactobacillus, a Sichuan pepper extract and a carrier. The feed additive is free from harmless residues and free of drug resistance; all raw materials are matched for use, so that the effect is good, and the stability is ensured. The preparation method is characterized in that the raw materials are mixed based on ratio; the operation is simple and quick; the condition controllability is high. After the feed additive is addedto a feed of the growing and fattening pigs, the appetite and the growing performance of the growing and fattening pigs can be improved; the meat quality of the growing and fattening pigs can be improved; and moreover, the intestinal flora of the growing and fattening pigs can keep balance; the immunity and the oxidation resistance of the growing and fattening pigs can be improved; and moreover,the nitrogen and phosphorus emission in the excrement of the growing and fattening pigs can be reduced; the feed waste is reduced; and the environmental pollution is reduced.