Abstract: To explore the effect of carotenoid structure on its free radical scavenging ability,quantitative chemical calculations were carried out on 13 kinds of carotenoids. After optimization of the structure by AM1,carotenoids was further optimized by density functional theory(DFT)at B3LYP/6-311(d,p).Four methods were used to determine the free radical scavenging capacity of β-carotene,lutein,astaxanthin and lycopene. The results showed that different carotenoids had different ability to scavenge different free radicals,astaxanthin had the strongest free radical scavenging ability,followed by lycopene,lutein,and β-carotene was the weakest. According to the frontier orbital energy difference and the distribution of NPA charge,the special structure of multiple conjugated double bonds of carotenoids was an important reason for its ability to scavenge free radicals,the groups on the terminal rings could promote the activity of the molecules. The keto group had the greatest effect on carotenoids’ activity,followed by the hydroxyl and epoxy groups,which was consistent with the experimental data,indicating that the frontier orbital energy difference and the distribution of NPA charge were important parameters to characterize the scavenging ability of carotenoids.