Abstract: The flavonoids of Lithocarpus polystachyus Rehd were extracted by ultrasound assisted method. The effects of liquid-material ratio,ethanol concentration,ultrasonic power,and ultrasonic time on the extraction yield were studied,and the response surface methodology was used to optimize the extraction process conditions. ADS-7 macroporous resin was utilized to separate the phlorizin,and the antioxidant activities of total flavonoids,phlorizin,and flavonoid R(the remaining flavones after phlorizin removal)were analyzed. The results indicated that the order of factors affecting the total flavonoid extraction yield was the ultrasonic time>ethanol concentration>ultrasonic power>liquid-material ratio. The optimized parameters were as follows:Liquid-material ratio 40∶1,ethanol concentration 80%,ultrasound power of 540 W,and ultrasound time of 60 min. Under these conditions,the total flavonoid yield was 8.82%±0.09%. The IC₅₀ values of DPPH free radical scavenging by total flavonoids,phlorizin,and flavonoids R were 0.0205,0.0222,and 0.0261 mg/mL,respectively. The IC₅₀ values of total flavonoids,phlorizin,and flavonoids R scavenging ABTS free radicals were 0.0220,0.0233,and 0.0266 mg/mL. The antioxidant capacity of total flavonoids was the strongest,followed by phlorizin and then by flavonoid R,all of which showed great DPPH and ABTS free radical scavenging ability.