In this paper， the degree of hydrolysis （DH） of casein and ACE inhibitory activity of hydrolysate and its gastrointestinal simulated digestion were studied during the enzymatic process. Besides， near-infrared on-line monitoring technology was used to collect the spectral information of the enzymatic process. The mathematic model was established by the relationship between the spectral information and the chemical value. The results showed that the degree of hydrolysis of casein and ACE inhibitory activity of casein hydrolysate increased with the prolongation of the enzymolysis time. And the ACE inhibitory activity of the hydrolysate after gastrointestinal simulated digestion also increased. The quantitative model was established between spectral information and the chemical value using partial least squares method， interval partial least squares method and joint interval partial least squares method. Results showed that joint interval partial least squares method could predict key parameters of enzymatic reaction and the mathematic model had good stability. In conclusion： near-infrared spectroscopy can be used to monitor the enzymatic reaction process on-line and to predict the DH of casein and ACE inhibitory activity of casein-hydrolysate and its gastrointestinal simulated digested product.