Abstract: In order to study the antibacterial activity of Astragalus extract on Staphylococcus aureus ,Escherichia coli and Salmonella sp., the antibacterial activity of Astragalus extract on S. aureus, E. coli and Salmonella sp. in vitro was determined by using tube double dilution method. Mice were administrated with the same concentration of pathogenic bacteria by intragastric administration to treat with Astragalus extract. The number of intestinal bacteria in the duodenum,jejunum,ileum and cecum of mice were respectively determined so as to study the antibacterial activity of Astragalus extract in vivo. The results showed that Astragalus extract had inhibitory effects on S. aureus, E. coli and Salmonella sp. in vitro and in vivo. And Astragalus extract had strongest inhibition on E. coli and weakest inhibition on Salmonella sp. The mortality rate of mice after infecting with various kinds of pathogenic bacteria was significantly reduced by perfusing Astragalus extract. In conclusion, Astragalus extract had obvious antibacterial effects on S. aureus ,E. cali and Salmonella sp. in vitro and in vivo.