Abstract: 【Objective】 To investigate the effects of alfalfa supplementation on nutrient apparent digestibility, rumen fermentation parameters and rumen microflora structure of Hu lambs aged from 15 to 55 days. 【Methods】 Forty-eight second-litter double-lamb lake lambs with body weight of (4.22±0.35) kg were randomly divided into 3 groups with 4 replicates per group. They were fed diets containing 10% (M1), 20% (M2) and 30% (M3) alfalfa, respectively. According to the different feeding and management of lambs, the experiment was divided into 15~34 days of age and 35~54 days of age. 【Results】 The results showed as follows: At 54 days of age, the body weight of 30% alfalfa group was higher than that of other experimental groups, but there was no significant difference (P > 0.05); At 15 to 34 days of age and 35 to 54 days of age, there were no significant differences in the apparent digestibility of dry matter, crude protein, neutral detergent fiber and acid detergent fiber among treatment groups (P > 0.05). There was no significant difference in pH among experimental groups (P > 0.05), NH₃-N content of 10% alfalfa group was significantly higher than that of 20% alfalfa group and 30% alfalfa group (P < 0.05), and the contents of total volatile fatty acids, acetic acid and butyric acid of 30% alfalfa group were significantly higher than those of 10% alfalfa group (P< 0.05). The activities of cellulase, xylanase and pectinase in 10% alfalfa group were significantly higher than those in 30% alfalfa group (P < 0.05); The Shannon index of 30% alfalfa group was significantly higher than that of 10% alfalfa group and 20% alfalfa group (P<0.05). The abundance of firmicutes in 20% alfalfa group and 30% alfalfa group was increased compared with 10% alfalfa group (0.05 < P≤0.10), but there were no significant differences in the levels of other rumen bacteria among experimental groups (P > 0.05). The abundance of Prevotella in 10% alfalfa group was significantly higher than that in other experimental groups (P<0.05), and the abundance of Eurosenia in 30% alfalfa group was significantly higher than that in other experimental groups (P<0.05). The number of bacterial amine and polyamine biosynthesis pathways in 20% alfalfa group was significantly higher than that in other experimental groups (P<0.05), and the number of aminoacyl-trNA pathway and aromatic compound biosynthesis pathway in 30% alfalfa group was significantly higher than that in 10% alfalfa group (P< 0.05). The number of metabolic regulator biosynthesis pathways in 30% alfalfa group was significantly lower than that in other experimental groups (P < 0.05). 【Conclusion】 According to the results, it is appropriate to add 20%-30% alfalfa in the diet of lambs aged from 15 to 55 days.