Objective:To study the effects of 24-epibrassinolide （EBR） at different concentrations on seed germination， seedling growth and key enzyme activities for flavonoid biosynthesis in Astragalus membranaceus var. mongholicus under polyethylene glycol （PEG）-induced drought stress， in order to provide theoretical reference for standardizing the drought-resistant cultivation techniques of A. membranaceus var. mongholicus. Method:The seeds of A. membranaceus var. mongholicus were soaked in EBR solution at different concentrations（0.001， 0.01， 0.1， 1， 10 μmol·L^-1） followed by foliar spraying to explore their effects on seed germination， seedling growth， photosynthesis， chlorophyll and malondialdehyde （MDA） contents， and key enzyme activities for flavonoid biosynthesis under drought stress induced by 20% polyethylene glycol （PEG）-6000. Result:Compared with the control（CK）group， PEG-induced drought stress led to an obvious decrease in germination potential， germination rate， germination index， vigor index， relative germination rate， plant height， root length， aboveground and root dry weight， net photosynthetic rate （Pn）， stomata conductance （Gs）， transpiration rate （Tr）， chlorophyll contents and chlorophyll a/b ratio， while a significant increase in intercellular carbon dioxide concentration （Gi）， MDA， flavonoids contents， and key enzyme activities for flavonoid biosynthesis such as phenylalanine ammonia lyase （PAL）. The treatment with exogenous EBR solution at the suitable concentration significantly enhanced the adaptation of A. membranaceus var. mongholicus seeds and seedlings to PEG-induced drought stress， manifested as significantly elevated germination potential， germination rate， germination index， vigor index， relative germination rate， plant height， root length， aboveground and root dry weight， Pn， Gs， Tr， chlorophyll a/b ratio， chlorophyll and flavonoids contents and key enzyme activities for flavonoid biosynthesis like PAL while lowered Ci and MDA contents. The optimal concentration of EBR solution was 0.1 μmol·L^-1. Conclusion:Exogenous EBR solution at the suitable concentration ameliorates the inhibitory effect of 20% PEG stress against seed germination and seedling growth of A. membranaceus var. mongholicus， reduces the oxidative damage in leaves， and improves the stress resistance to a certain extent by up-regulating the key enzyme activities and promoting flavonoid synthesis.