The invention relates to the technical field of medicine, and provides a flavonoid new compound-common seepweed herb flavone A with the immunosuppression activity, and the chemical name is 5,6,8-triethoxy-7-hydroxyc coumaronochromone(5,6,8-trimethoxy-7-hydroxycoumaronochromone), and the structural formula is shown in the specification. The invention further relates to a preparing method of the compound and the function of the compound on lymphopoiesis induced through LPS and ConA. The study result shows that the common seepweed herb flavone A has good activity of restraining lymphopoiesis, the IC50 value can reach 0.39 [mu]M, and the common seepweed herb flavone A can be used for preparing medicine treating diseases related to immunity.