Radix Astragali (Astragalus membranaceus) is an important traditional Chinese medicine that is widely used as a tonic to enhance the body's natural defense mechanisms. In this phytochemical study, 12 flavonoids, isoliquiritigenin (1), liquiritigenin (2), calycosin (3), calycosin 7-O-β-D-glucoside (4), formononetin (5), formononetin 7-O-β-D-glucoside (6), daidzein (7), daidzein 7-O-β-D-glucoside (8), methylnissolin (9), methylnissolin 3-O-β-D-glucoside (10), isomucronulatol (11), and isomucronulatol 7-O-β-D-glucoside (12), were isolated from the roots of A. membranaceus. Their structures were elucidated by comparing spectroscopic data with reported values. The effects of the isolated compounds on lipopolysaccharide (LPS)-stimulated bone marrow-derived dendritic cells were investigated. Compounds 1 and 2 exhibited significant inhibitory effects on LPS-induced IL-6 and IL-12 p40 production, with IC50 values ranging from 2.7 to 6.1 μM. Compound 1 also showed a moderate inhibitory effect on LPS-stimulated production of TNF-α with an IC50 value of 20.1 μM. Further studies of the potential anti-inflammatory effects and benefits of flavonoids from A. membranaceus are warranted.