Abstract: Preparation of yak milk casein peptides by hydrolysis of yak milk casein with alkaline protease was studied. Yak milk casein peptides was modified to carry out on the debitterizing research by the acetic anhydride of structural modification.The influence of the adding amount of acetic anhydride and pH on the degree of Yak milk casein peptides acetylation and bitterness was discussed. The result showed that:the enzyme dosage of substrate 1.25%, pH7.0, 55℃, 3h had the optimum hydrolysis conditions. Maintaining acylation reaction pH above 7, the degree of acetylation were the highest, the acetic anhydride adding amount proportional to the effects of debittering. When acetic anhydride dosage was 40%, the bitterness of yak milk casein peptide acylation product would be disappeared.