Potato protein was hydrolyzed by trypsin to prepare for ACE inhibitory peptides in our previous researches (the ACE inhibition rates was 61.20%±0.15%), and we used plastein reaction to modify potato protein hydrolysates for enhancing the ACE inhibitory activity of peptides in this study. The influences of the kinds of added amino acids, the substrate/enzyme ratio, pH value, the temperature and hydrolysis time on ACE inhibitory activity of the plastein reaction modification of potato protein hydrolysates were investigated. To achieve the maximum ACE inhibitory activity in the plastein reaction modification of cottonseed protein hydrolysates, the response surface methodology was applied to find the optimum conditions based on the single factor tests. \[E\]/\[S\] ratio at 3 565.05 U/g, substrate concentration at 30.0%, and extrinsic dose of glycine at 0.5 mol/g reacted at 43.68 ℃ and pH 8.05 for 3.0 h, were found to be the optimal conditions, and the ACE inhibitory activity could reached (82.89±0.05)%. Compared with unmodified potato ACE inhibitory peptide, its ACE inhibitory activity increased by 1.35 times.