That soy protein isolate hydrolysis by alkali protease can produce soybean peptide.To judge its hydrolysis efficiency,amino nitrogen content in the enzyme solution is determined with triketohydrindene hydrate colorimetric method.The factors such as enzyme quality,pH,substrate concentration,the temperature of reaction,the time of reaction are discussed in emphasis.The optimal conditions of hydrolysis were determined by single factor and orthogonal experiments.The optimized conditions of alkling protease hydrolysis are 50℃,pH value 8.5,7% of enzyme addition,1∶15 (solid-to-liquid ratio) of substrate concentration,5h of hydrolysis.