Abstract:The characteristics of the product in the process of digestion with trypsin after the enzymolysis of the ultrasonic pretreated casein were studied. The particle size distribution of caseian were measured with laser particle size distribution analyzer， and the surface characteristics of casein were detected by the electronic scanning electron microscopy（SEM）， and the secondary structure changes in casein were measured with Fourier infrared spectrum. The results showed that it was easy to make holes or layers in the dense surface structure of ultrasonic pretreated casein than the untreated casein. The larger casein particles changed into smaller one and molecular weight distribution became more uniform. These changes made the originally dense surface structure of casein to become relatively incompact and increase the specific surface area. These changes were conducive to the subsequent enzymolysis. The beta angle and random curl reduced as well as the contents of beta angle increased in casein after enzymolysis. The results showed that it had a good effect on the enzymatic hydrolysis of casein with ultrasonic assisted treatment.