Abstract:The ultrafiltration method for preliminary separation and purification of the antioxidant activity peptide from alfalfa soluble protein hydrolysates(ASPHs) was introduced.The effects of ultrafiltration on the molecular weight distribution,amino acids composition and the antioxidant activity of ASPHs were investigated.The results show that the optimum conditions of ultrafiltration are concentrations of ASPHs 35 g/L,pressure of 0.35 MPa,and temperature of 25 ℃.The molecular weight distribution of ASPHs in 1 000~500,500~130 and under 300 increase from 21.85%,21.64% and 4.91% to 27.91%,30.83% and 7.61% respectively after ultrafiltration.The antioxidant activity of ASPHs has also increased.