| Summary: | This study determined the optimum reaction conditions for synthesizing selenium-containing polysaccharides. Polysaccharide IIA (with the highest yield) from <i>Ornithogalum caudatum</i> Ait. (Liliaceae) (OCAPIIA) was extracted and purified. Then, three parameters were selected to optimize the synthesis of selenized OCAPIIA (Se-OCAPIIA) using the Box–Behnken design (BBD) and response surface methodology (RSM). The morphology of Se-OCAPIIA was analyzed by scanning electron microscopy (SEM). The characteristic peaks and the monosaccharide composition of Se-OCAPIIA were evaluated by Fourier-transform infrared spectroscopy and gas chromatography. A D-galactose-induced aging mouse model was established, and the in vivo antioxidant activity of Se-OCAPIIA was measured. The optimal conditions for the synthesis of Se-OCAPIIA were as follows: reaction temperature, 72.38 °C; Na<sub>2</sub>SeO<sub>3</sub> to OCAPIIA mass ratio, 0.93 g/g; and reaction time, 8.05 h. The selenium content of Se-OCAPIIA obtained using the optimized process was 3.131 ± 0.090 mg/g, close to the maximum predicted value (3.152 mg/g). Se-OCAPIIA contained D-mannose, D-glucose, and D-galactose at a molar ratio of 1.00:0.34:0.88. SEM showed that Se-OCAPIIA was spherical and flocculated. Compared with OCAPIIA, Se-OCAPIIA exhibited two characteristic peaks at 833 and 610 cm<sup>−1</sup> in the infrared spectrum. Se-OCAPIIA increased catalase, glutathione peroxidase, and superoxide dismutase activities and decreased MDA concentrations in the mouse liver. Moreover, Se-OCAPIIA treatment improved liver morphology, decreased the levels of IL-1β and IL-6, and increased IL-10 concentration. In conclusion, the synthesis of Se-OCAPIIA is effective, simple, and feasible. Se-OCAPIIA demonstrated high antioxidant activity in vivo and is a promising antioxidant and therapeutic agent.
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