Abstract:Total flavonoids of Astragalus membranaceus stems and leaves were obtained by ultrasonic-assisted extraction in an ethanol-ammonium sulfate aqueous two-phase system.With the extraction amount of total flavonoids as an index，the extraction process was optimized by combining the single-factor experiments with response surface analysis，and the antioxidant activity of the total flavonoids was also investigated. The results showed that the optimal conditions for ultrasonic-assisted aqueous two-phase extraction of total flavonoids from Astragalus membranaceus stems and leaves were as follows：ethanol volume fraction of 60%，ammonium sulfate concentration of 0.20 g/mL，liquid-solid ratio of 31∶1（mL/g），ultrasonic time of 50 min，ultrasonic temperature of 46 ℃，and ultrasonic power of 480 W. Under these conditions，the amount of total flavonoids was 15.008 mg/g. The prepared total flavonoid extracts from Astragalus membranaceus stems and leaves exhibited good antioxidant activity. The IC50 values for scavenging DPPH·，ABTS+·，and ·OH were 0.933，0.691，and 0.965 mg/mL，respectively.When the concentration of the total flavonoids was 2.0 mg/mL，its ferric ion reducing antioxidant power（FRAP）value was 1.384 mmol/L，which was 83.47%of that of the positive control VC at the same concentration.