To investigate the active substances in the seeds of euphorbia lathyris, its ethyl acetate extract was obtained after the seeds were powdered, ethanol immersion extracted, condensed, degreased and extracted with ethyl acetate. The monomer compound was obtained after the ethyl acetate extract was washed and recrystallized.The compound was identified as 6,7-dihydroxycoumarin by mass-spectrometry and nuclear magnetic resonance，whose antioxidant activity was evaluated by DPPH radical scavenging assay and antioxidant experiment on lard. The result showed that the IC50 values of 6,7-dihydroxycoumarin was 0.058 μg/mL, much lower than that of Vitamine C, 0.542 μg/mL. The result of lipid peroxide value test showed that the antioxidant activity of 6,7-dihydroxycoumarin was stronger than Vitamine C.