The kinetics of protein hydrolysis catalyzed by alkaline protease Alcalase 2.4L was investigated in the peanut emulsion medium. The results showed that degree of protein hydrolysis was enhanced with the increase of initial enzyme concentration, but it was decreased with the increase of initial substrate concentration at fixed enzyme concentration. There was a good positive relationship between degree of protein hydrolysis and demulsification of emulsion (r=0.983). The apparent Michaelis constant Km was 0.0698mol/L and the maximum velocity Vmax was 3.71×10-4mol/(min·L) in the peanut emulsion. The kinetic equation of peanut protein hydrolyzed by Alcalase 2.4L at initial stage was obtained based on the experimental data. The critical substrate concentration was 8.73g/L (initial enzyme concentration was 0.05%) for hydrolysis of protein and demulsification.