Abstract: The influence of ultrasonic on the molecular weight and distribution of papain hydrolyzate was discussed in this paper.The enzymolysis products of free and immobilized enzymes were determined by high performance liquid chromatography ( HPLC) with bovine serum albumin as substrate.The results showed that there were significant differences in the molecular weight and distribution of the hydrolyzate peptide between free and immobilized enzymes under ultrasound and non-ultrasound conditions.The molecular weight and distribution of free papain and immobilized papain hydrolyzate under ultrasonic conditions were found to be mainly distributed in the range of 180¹000 Da and 1000⁵000 Da.The proportion of hydrolyzed peptides in immobilized papain under the condition of < 180 Da was relatively large.