The invention discloses a method for preparing antioxidative peptide by using cod to process a by-product. A cod processed by-product cod fillet is taken as a raw material, the cod fillet having rich protein is subjected to high- temperature and high pressure boiling and crushing, water is added for preparing a cod fillet solution, the pH value is adjusted, pancreatin is added and is hydrolyzed under constant temperature, enzyme deactivation is carried out after the reaction is completed, refrigerated centrifugation is carried out, a supernatant is taken, and then is subjected to the steps of ultrafiltration, concentration and freeze drying to obtain the cod fillet antioxidative peptide powder. The obtained peptide powder has good removing rate on DPPH free radical, hydroxyl radical and superoxide anion, and is an excellent anti-oxidant. The peptide powder has the characteristics of light yellow, no bitter taste, and slight fishiness, the molecular weight can be controlled to less than or equal to 1000 Da, and the polypeptide total protein is 89.56%. The used cod fillet is the cod processed by-product, the cost is low, the prepared antioxidative peptide has the advantages of good antioxidation effect, high security, and no toxic and side effect, and high-quality antioxidative peptide is provided for food, cosmetic and medicine industries.