Abstract: In order to study the effect of alfalfa silage instead of the same amount of alfalfa hay on production perfermance, urea nitrogen and blood biochemical index of dairy cows, the big pit silage experiment adding 5% molasses to alfalfa and feeding test were carried out. The results showed that the replacement of alfalfa hay with alfalfa silage were beneficial to improve DMI (Dry Matter Intake), milk yield and milk fat although milk protein decreased in the later stage without significance(P0.05). The contents of MUN（Milk Urea Nitrogen）and UUN(Urinary Urea Nitrogen) decreased for the both groups while the content of BUN(Plasma Urea Nitrogen) firstly decreased and increased. The contents of MUN and BUN from tested groups (maize silage + alfalfa silage) were lower than those from control (maize silage + alfalfa silage hay) without significance whereas the content of UUN performed reversely. In the later stage, the content of UUN from experiment group and control group were 48.35 and 27.52 mmolL-1, respectively, which had significant difference (P0.05). The contents of ALT(Alanine Aminotransferase）、AST（Aspartate Transaminase） and ALP（Alkaline Phosphatase） from tested groups were higher than those from control without significance. The contents of ALB (Albumin） and GLU (Glucose） from tested groups gradually decreased which were lower than those from control without significance. The content of CHO (Chinese Hamster Ovary） from tested group gradually increased and that from control group gradually decreased which reached the biggest difference at 3.64 and 2.82 mmolL-1 (P=0.059), respectively. In conclusion, alfalfa silage feeding was beneficial to improve production perfermance and metabolism ability because the effect of metabolism and utilization of alfalfa silage was reduced in later experiment by Maillard Reaction,