Abstract: In this experiment, three functional oligosaccharides (glucoosaccharides, raffinose, trehalose) and sodium selenite were used as raw materials to prepare selenide oligosaccharides by HNO₃-Na₂SeO₃ method. The structures of three functional oligosaccharides and their selenide oligosaccharides were characterized by UV and IR spectra. The effect of selenium modification on the antioxidant activity of oligosaccharides was studied by in vitro antioxidant activity test. The selenium contents of selenose-oligosaccharide, selenoraffinose and selenotrehalose prepared under the experimental conditions were 10.450, 10.514 and 10.488 mg/g, respectively. The structural characterization results showed that the three selenide oligosaccharides all contained the characteristic peak of selenite Se=O, and the basic skeleton of the oligosaccharides was preserved, indicating that the three functional oligosaccharides were successfully selenized by HNO₃-Na₂SeO₃ method. In vitro antioxidant activity experiment showed that after selenization modification, the DPPH free radical scavenging rate of glucooligosaccharide, raffinose and trehalose was slightly increased, while the hydroxyl free radical and ABTS cationic free radical scavenging effect of the three was significantly improved. When the sample concentration was 10 mg/mL, the hydroxyl radical scavenging rates of selenooligosaccharides, selenoraffinose and selenotrehalose were about 84.13%, 98.17% and 98.17%, respectively, and the cation free radical scavenging rates of ABTS were about 40.47%, 48.27% and 90.40%, respectively. The scavenging rates of hydroxyl free radicals and ABTS cationic free radicals of trehalose selenide were similar to that of V_C at corresponding concentrations. In conclusion, the scavenging activity of hydroxyl radical and ABTS cationic radical of oligosaccharides can be significantly improved by selenization modification, which can provide reference for the development of selenium-supplementing preparations and functional foods related to selenide oligosaccharides.