OBJECTIVE：To compare the protective effects model mice between the aboveground and underground parts of Astragalus membranaceus on immunosuppression ，and to provide reference for further utilization and development of A. membranaceus. METHODS ：A total of 240 ICR mice were divided into 4 batches，60 mice in each batch ，with half male and half female. Each batch of mice were randomly divided into blank group ，model group ，A. membranaceus aboveground part and undergroud part low-dose and high-dose groups （3，6 g/kg，by crude drug ）according to body weight and sex ，with 10 mice in each group. Blank group and model group were given normal saline intragastrically. A. Membranaceus groups were given corresponding concentration of drug intragastrically ，10 mL/kg，once a day ，for consecutive 30 days. Except for blank group ， other groups were intraperitoneally injected with cyclophosphamide 40 mg/（kg·d）for consecutive 3 days，since 24th day of treatment，to establish immunosuppression model. The levels of serum immunoglobulin （IgG，IgM，IgA），inflammation factors [nitric oxide ，interleukin-2（IL-2），IL-6，tumor necrosis factor-α] and half hemolysis value were detected in each group. Body weight ，thymus index ，spleen index ，phagocytic index ，activity of natural killer （NK）cell，splenic lymphocyte proliferation ability，dinitrofluorobenzene-induced delayed metamorphosis reaction in mice （by weight difference between left and right ears ） and the number of hemolytic plaque were determined. RESULTS ： Compared with blank group ， the serum levels of immunoglobulin，body weight ，thymus index ，spleen index ，phagocytic index ，NK cell activity ，the proliferation ability of splenic lymphocyte，the number of hemdytic plaque and half hemolysis value were decreased significantly in model group （P＜0.05）， while inflammation factor level as well as weight difference between left and right ears were increased significantly （P＜0.05）. Compared with model group ，above indexes of mice in A. membranaceus groups were improved significantly ，in dose-dependent manner（P＜0.05）. Compared with A. membranaceus undergroud part group ，above indexes of A. membranaceus aboveground part group were improved significantly （P＜0.05）. CONCLUSIONS ：Aboveground and underground part of A. membranaceus both have pretective effect on immunosuppression model mice ，and the effect of aboveground part of A. membranaceus is stronger than underground part of A. membranaceus .