Alaska pollock frame is a kind of byproduct that was rich in protein, amino acids, and mineral elements. However, the unfavorite bitterness may probably be produced in enzymatic processes. In this study, the bitterness accounted from the hydrolysates prepared by neutral proteases, alkaline proteases, papain, flavourzyme, and animal proteases, was investigated. The hydrolysis conditions, amino acids composition, metal ion composition, molecular weight distribution, and peptide composition of the hydrolysates were detected to figure out the relationship between bitterness and compositions of the hydrolysates. The hydrolysate digested by alkaline protease has the highest bitterness intensity, and that amino acids composition, peptide composition, and molecular weight distribution had a significant influence on the bitterness degree. Hydrophobic amino acids and alkaline amino acids, such as leucine, isoleucine, lysine, and so on are likely to contribute to the bitterness and molecular weight distribution of peptides that affect bitterness is mainly lower than 3,000 Da. PRACTICAL APPLICATIONS: Fish bones would produce a bitter taste when reusing them by hydrolysis. Bitterness is one of unfavorable flavor as to consumers. The results of this study are of great significance for the further utilization of Alaska pollock frame. For products obtained from the hydrolysate of Alaska pollock frame, such as condiments and health care product the results of this study provide the processing technology of the lowest bitter hydrolysate, which can effectively improve the flavor and acceptability of the products.