Abstract: In order to investigate the effects of different concentrate to forage ratio diets on serum enzyme activity, immune performance and muscle composition of Hu sheep, forty healthy Hu sheep were randomly divided into four groups with ten sheep in each group, half male and half female. The sheep in the four groups were fed a complete mixed diet with concentrate to forage ratio of 70∶30（test group Ⅰ）, 60∶40（test group Ⅱ）, 50∶50（test group Ⅲ） and 40∶60（test group Ⅳ）, respectively. The experiment lasted for 70 days. On day 14, 35, and 56 of the experiment, six Hu sheep in each group were randomly selected to collect 5 mL blood from the fasting jugular vein. The isolated serum was used to determine the activity of serum enzymes（aspartate aminotransferase AST, alanine aminotransferase ALT, creatine kinase CK, alkaline phosphatase ALP, cortisol COR） and immune performance indexes（immunoglobulin A IgA, immunoglobulin protein G IgG and immunoglobulin protein M IgM）. After the feeding experiment, six Hu sheep in each group were selected and slaughtered. The longissimus dorsi muscles were collected for determination of the contents of amino acids and fatty acids by liquid chromatograph. The results showed that on the 14th day of experiment, the serum ALT activity in the test group Ⅰ was significantly higher than that in the test group Ⅳ（P＜0.05）, and the AST/ALT in the test group Ⅳ was significantly higher than that in the test groups Ⅰ and Ⅱ（P＜0.05）. On the 35th day, the activity of ALT in serum of lake sheep in the test group Ⅰ was significantly higher than that in the test group Ⅳ（P＜0.05）, the AST/ALT in the test groups Ⅰ and Ⅳ was significantly higher than that in the test groups Ⅱ and Ⅲ（P＜0.05）, and the activity of CK in the test group Ⅰ was significantly higher than that in the test groups Ⅱ and Ⅲ（P＜0.05）. The serum ALP activity of Hu sheep in the test group Ⅲ was significantly higher than that in the test group Ⅰ（P＜0.05）. On the 56th day, the serum ALT activity of Hu sheep in the test group Ⅳ was significantly lower than that in the test group Ⅱ（P＜0.05）. On the 14th day of the experiment, the serum IgG concentration of Hu sheep in the test group Ⅲ was significantly higher than that in the test groups Ⅱ and Ⅳ（P＜0.05）. On the 35th day, the concentrations of IgG and IgA in serum of Hu sheep in the test group Ⅳ were significantly lower than those in the test group Ⅲ（P＜0.05）. On the 56th day, the serum IgM concentration of Hu sheep in the test group Ⅳ was significantly lower than that in the test group Ⅰ（P＜0.05）. There were no significant differences in total amino acid（TAA） among all groups（P＞0.05）, but the content of Asp, Glu, Ser and Arg in the test group Ⅳ were significantly higher than those in the test group Ⅲ（P＜0.05）, the content of Met was significantly higher than that in the test group Ⅰ（P＜0.05）. There were no significant differences in fatty acid contents among all groups（P＞0.05）, but most fatty acid contents in the test group Ⅰ were higher than those in other groups. The results indicated that the low concentrate to forage ratio（40∶60） diet could improve serum enzyme activity and immune performance and the contents of some amino acids in muscle, but had no effects on the contents of fatty acids in muscle.