Abstract: First, compared the solubility, nitrogen solubility index and secondary structure of three commercial soy protein isolates, and employed the soy protein isolate marked sample 1 as selected protein of future research on optimization.Based on Box- Behnken response surface method, the amount of trypsin, reaction temperature, reaction time and reaction pH were chosen as influencing factors and emulsifying active index was selected as response value.The optimum conditions were as follows:the amount of trypsin was 2.72%, reaction temperature was 40℃, reaction time was 4.12h and reaction pH was 8.85.The optimal emulsifying active index was 22.57m2/g.