The aim of this study was to evaluate the effects of compound probiotics on milk performance,blood biochemical indexes,rumen fermentation parameters and nutrient digestibility of Holstein dairy cows in middle-and-later lactation.Forty-eight dairy cows with similar milk yield,days in milk and age were randomly divided into 4 groups treated with four different levels of compound probiotics as follows:the control group (0 g/d),group 1 (10 g/d),group 2 (20 g/d) and group 3 (30 g/d).The adaptation period was 7 days,and the total experimental period was 60 days.Results showed that:compared with the control group,supplementation of compound probiotics could improve milk yield and significantly increase the concentration of milk protein,lactose and total solids (P＜0.05);there was no significant effect on blood biochemical indexes (P＞ 0.05);compound probiotics could significantly increase the content of ammonia and microbial protein content (MCP) in the rumen (P＜ 0.05);the apparent nutrient digestibility of crude protein and crude fat of group 2 was significantly higher than other groups (P＜0.05);compared with the control group,the economic benefits of group 1,2,3 increased 2.28,4.80 and 4.09 yuan/d,respectively.In summary,dietary supplementation with 20 g/d of compound probiotics was the most effective method for milk performance,rumen fermentation parameters,nutrients apparent digestibility and economic benefit of Holstein dairy cows.