The invention relates to a hydrolyzed fish protein powder and a preparation method thereof, belonging to the technical fields of food protein and food protein processing. The preparation method of the hydrolyzed fish protein powder comprises the following steps: firstly determining a raw fish material used for hydrolysis; then hydrolyzing by virtue of protease of the raw fish material; and finally heating hydrolyzate to inactivate the protease, extracting the hydrolyzate, concentrating, and drying, thus the hydrolyzed fish protein powder is obtained. The hydrolyzed fish protein powder comprises protein, hydrolyzed protein, polypeptide, free amino acid, fat, carbohydrate and mineral substances, wherein the ratio of the total content of the hydrolyzed protein in the hydrolyzed fish protein powder to the total content of the protein and the ratio of the total content of the free amino acid to the total content of the hydrolyzed protein are high, and oligopeptide is also contained in the hydrolyzed fish protein powder. The invention has the advantages that in the method, human resource can be saved and waste of the raw fish material is avoided, thus being convenient for mechanized production; meanwhile, the hydrolysis process is efficient and coordinated, the quality of hydrolyzed product is high, especially the content of the free amino acid and oligopeptide is high. The hydrolyzed fish protein powder product in the invention is a human nutrition enriching supplement which is prepared from fresh water fish raw material, contains no additive, preservative, organic solvent or man-made addition material, has high water solubility and high free amino acid content and is natural.