Aim: We aimed to monitor an acid polysaccharide with antioxidant activity. Materials and Methods: Three acidic polysaccharides (DAP-A40, DAP-A60, and DAP-A80) were extracted from D. aurantiacum imbricate by alkaline aqueous solution extraction and ethanol precipitation. The structure characterization and antioxidant activity in vitro and in vivo were examined. Results: The molecular weight (MW) of three polysaccharides was 1.70 mol5 Da, 5.78 mol4 Da, and 1.36 mol4 Da, respectively, by high-performance gel chromatography. The consequences of infrared spectrum displayed that three polysaccharides enclosed uronic acid and sulfuric acid groups. DAP-A40 and DAP-A60 were collected of α-configuration pyranose. DAP-A80 was composed of β-configuration pyranose. Antioxidant test exhibited that DAP-A80 had outstanding antioxidant activity. The half-maximal inhibitory concentration of DAP-A80 in dipping power and scavenging the 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), DPPH, and OH radicals was 163.5 μg/mL, 613.2 μg/mL, 583.2 μg/mL, and 365.1 μg/mL, respectively. The grades of correlation analysis showed that the ability of DAP-A80 to scavenge ABTS, DPPH, and OH radicals was negatively connected with the MW of polysaccharides and certainly correlated with the content of sulfate group. The results of in vivo experiments exposed that DAP-A80 might knowingly decline the formation of malondialdehyde. High levels of DAP-A80 could evocatively upsurge the activity of superoxide dismutase and GSH-Px in mouse serum. Conclusion: DAP-A80 has an antioxidant effect and can be reconnoitered as a health medicine and food.