Abstract: The enzymatic limited hydrolysis of soy protein isolate (SPI) by Alcalase 0.6L was carried out with pH-stat method at pH 8.0,50℃.The effects of the main parameters on the degree of hydrolysis, x , were investigated and a model based on the experiment results was deduced.The results showed that the substrate and the enzyme-substrate intermediate induced enzyme inhibition,which occurred due to free enzyme attacking the enzyme-substrate complexes.A critical 0 would correspond to a constant 0,which was 96.77mg·ml ^-1 under the experiment condition.The kinetic parameters calculated from the experiment data were K _m=0.016mol·L ^-1 and V _max =0.765mol·min·L ^-1 .The results showed that x can be regulated to about 6.0 with the mentioned factors in 30min.The good fitness between predicted and measured results confirmed the usefulness of the model.