The invention belongs to the field of feed additives, and particularly relates to active peptide for feed and a preparation method. The preparation method of the active peptide for the feed comprises the following steps: pulping fresh fish leftovers, mixing fish pulp and soybean meal, enzymatic hydrolyzing, deactivating enzyme, filtering, decolorizing, removing fishy smell, and performing spray drying. The active peptide prepared according to the preparation method is high in small peptide content, high in digestibility and balanced in amino acid ratio; as a feed additive for feeding animals, the active peptide for the feed is good in palatability and high in nutritional value, can improve the animal immunity and can substitute a part of fish meal and blood plasma powder.