[Objective] The healthy growth of weaned calves is the premise to ensure the healthy lactation ability of dairy cows. The compound microecological agents have good application value in animal breeding,so it is of great significance to explore the effects of compound microecological agents on growth performance,diarrhea and serum related biochemical indexes of weaned calves for the healthy development of dairy farming. [Method] 32 weaned Holstein calves with the same body weight at about 50 days of age were randomly divided into trial group A（control group）,trial group B,C and D with 8 weaned calves in each group. The control group was fed with basic diet,while the trial group B, C and D were fed with 1 g,3 g and 6 g compound microecological agents respectively on the premise of basic diet,with a pre-feeding period of 7 days and a normal formal feeding period of 21 days. [Result] Compared with the control group,the body weight of weaned calves fed with compound microecological agents was effectively improved,and the digestibility of dry matter,crude protein and crude fat was increased significantly（P<0.05）.The diarrhea rate of weaned calves fed with compound microecological agents was effectively reduced. In biochemical serum indexes,T-AOC,GSH-Px and SOD indexes were increased,while MDA content was decreased in weaned calves fed with compound microecological agents. The expression of IgA,IgM and IgG related to immune ability was detected,and it was found that the expression was increased in weand calves fed compound microecological agents,and there was a significant difference compared with the control group（P<0.05）. [Conclusion] The compound microecological agents could effectively improve the growth performance of weaned calves,reduce the diarrhea rate,and improve the antioxidant capacity and immune capacity of the body.