Abstract: Six kinds of proteases were used to hydrolyze grass carp skin in order to select two optimum proteases by determining DPPH radical scavenging capacity and degree of hydrolysis( DH).Response surface methodology( RSM) was used to optimize hydrolysis conditions of two optimum proteases.Based on the results of RSM,the double enzyme hydrolysis tests were carried out to find out the best hydrolysis conditions.The results showed that hydrolysis conditions for alcalase were enzyme addition amount of 6.3%,material- liquid ratio of 1 ∶40.6,hydrolysis temperature of 51.3 ℃,and hydrolysis time of 115.1 min,and the scavenging activity on DPPH free radicals under the above conditions reached 54.98%.The optimal hydrolysis conditions for papain were 4.6%,1 ∶41.2,50.5 ℃ and133.0 min,and the scavenging activity reached 41.06%. By combined and sequential use of alcalase followed by papain,the DPPH scavenging activity was 92.07%,which was more effective than either of them.