Abstract: Fish scale is one of the fish processing leftover, with abundant nutrients like protein and mineral elements, which makes it a usable biological resource. This paper study the preparation of ACE inhibitory peptides by alkaline protease hydrolysis of the fish scale. The enzymatic hydrolysis conditions for the preparation of ACE inhibitory peptides were optimized using the response surface method based on single factor experiments, including hydrolysis temperature, hydrolysis pH, enzyme dose and substrate concentration. The results showed that the optimal enzymatic hydrolysis conditions were as follows: enzyme dose 61% (about 12 000 U/g), pH 89, hydrolysis temperature 547 ℃ for 2 hours. Under these conditions the theoretical value of ACE inhibition rate was 8536%, and the actual ACE inhibition rate was 862%, in which the relative error was 091%.