After blade grinding and screening pretreatment,the undersize brewer's spent grain was dealt with ball milling.Consequently,the content of crude protein was increased up to 38.8%,which was 36.1% higher than that in origin sample,while the content of soluble protein was up to 2.53%,which was 3.1 times higher than that in origin sample.The soluble protein content of undersize brewer's spent grain was increased up to 12.85% after the enzymatic hydrolysis of neutral protease,which was 1.6 times higher than that in origin sample(7.88%).The results showed that the efficiency of enzyme hydrolysis had been significantly improved by mechanical pretreatment.The enzyme hydrolysis conditions were optimized by single factor experiment and uniform design.The optimal conditions were as follows:the dosage of enzyme of 2400 U /g,enzyme hydrolysis temperature of 45 ℃,enzyme hydrolysis pH of8.0,and enzyme hydrolysis time of 8 h.Under these conditions,the content of soluble protein was up to 21.31%.The exaction rate was up to 54.9%.The hydrolysis liquid was dealt with vacuum freeze drying to prepare feed protein peptide.The crude protein content of feed protein peptide was up to 78.8%.The amino acids content was increased to 74.6%.Essential amino acids content was 28.18%,and polypeptides content was 60.8%.