Abstract: OBJECTIVE To study the effect of Astragalus polysaccharide(APS) on the proliferation level and the protective effect on DNA damage of human bone marrow mesenchymal stem cells which is induced by the X-ray irradiation. METHODS CCK-8 assay was used for screening of the optimal concentration of APS on HMSC-bm cells; the irradiation group was treated with 2.0 Gy doses of X radiation, while the medicated group was administrated with the optimal concentration of drug; cytokinesis-block micronucleus assay (CB micronucleus assay) was used to detect the micronucleus rate after X-ray irradiation; immunofluorescence test was used to detect the clustered 53BP1 foci. RESULTS APS of 50 μg·mL^-1 had the strongest stimulating effect on proliferation, which was used as the optimal drug concentration. Compared with the control group(0 μg·mL^-1 APS), the medicated group showed a significantly higher proliferation rate (P<0.05). After irradiation, the level of cell proliferation was significantly decreased (P<0.05). Compared with irradiation group, the irradiation plus drug group showed increased cell proliferation level(P<0.05), which was close to that of the control group. 53BP1 foci level increased signicicantly 0.5 and 2 h post irradiation (P<0.05), while 50 μg·mL^-1 APS reduced the 53BP1 level induced by irradiation. Compared with the control group, cell micronucleus rate increased(P<0.05) after X-ray irradiation, while it was reduced upon 50 μg·mL^-1 APS treatment(P<0.05). CONCLUSION APS has protective effect on X-ray radiation induced cell genomic DNA damage.