Abstract: Effects of metal endopeptidase on protein degradation of alfalfa silage was investigated in this study. After adding metal protease inhibitor EDTA in the previously fermented alfalfa juice (PFJ), samples of each treatment were opened on day 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 7 and 14 d and the protein components were analyzed. Results showed that non-protein nitrogen (NPN), NH3-N, amino acid nitrogen (AA-N) and pH value in EDTA treatment significantly decreased during ensiling process, but lactic acid concentration increased. On day 14, the concentrations of NPN, NH3-N and AA-N of EDTA treatment were 483.98 g/kg total nigrogen (TN), 256.98 g/kg TN and 26.11 g/kg TN, which decreased by 28.92%, 38.59% and 67.08% respectively compared with the control. Metal endopeptidase plays a role on the degradation of NPN during proteolysis process of ensiled alfalfa to some extent. Adding metal endopeptidase inhibitor can improve the fermentation quality of alfalfa silage.