Electron spin resonance( ESR) was used to study the antioxidant properties of flavonoids in four Pueraria extract samples( CEP1,CEP2,CEP3,CEP4) and Visible spectrophotometry was also used as comparison. Four samples mixed with DPPH solution were placed in electron paramagnetic resonance spectrometer and detected by Visible spectrophotometer. In the ESR assay,the different reaction time,with a clearance rate and quality of sample concentration on DPPH radical proportional relationship in the concentration of 60 mg /L,the free radical scavenging rates were significantly improved. ESR treatment time was 30 min,radical scavenging rate of Pueraria lyophilized powder eluated by 60% ethanol can reach to 100%. ESR method is more sensitive and more accurate in the determination of free radical scavenging effect. This study provides a scientific basis for the extraction and research of antioxidants.