Inoculation experiments were carried out in wheat-astragalus mixed cropping system to study the changes of plant biomass, nitrogen content in plants and soil, and enzyme activity of Rhizobium. The results showed that in the mixed cropping system inoculated with Rhizobium, the growth of plants was stimulated, total nitrogen of plants and soil was increased significantly, and the increment of enzyme activity of Rhizobium was also found.