Abstract A water-soluble polysaccharide (PSRC) was extracted and purified from the roots of Radix Cyathulae officinalis Kuan, and its chemical characteristics, monosaccharide composition and antioxidant activities were characterized. The average of molecular weight (Mw) of PSRC was 182kDa. The majority of monosaccharide components of PSRC was glucose (relative mass 48.8%) with lower levels of mannose, rhamnose, galactose, fructose and arabinose (relative mass of 15.7, 14.3, 10.6, 6.1 and 4.5%, respectively). In vitro assays revealed that RSRC possessed potent scavenging activities against DPPH, hydroxyl and superoxide anion radicals. Oral administration of PSRC significantly enhanced antioxidant enzyme activities (including total superoxide dismutase, copper-zinc superoxide dismutase (Cu,Zn-SOD), manganese superoxide dismutase (Mn-SOD), glutathione peroxidase and catalase (CAT)) and capacities of scavenging superoxide anion and hydroxyl radicals, markedly lowered lipid peroxidation formation of malondialdehyde and significantly up-regulated mRNA expressions of Cu,Zn-SOD, Mn-SOD, CAT, glutathione peroxidase 1, thioredoxin 1 and thioredoxin 2, in a d-galactose-induced aging mouse model. Taken together, these findings demonstrate that PSRC could be used as a novel promising source of natural antioxidants and antiaging drugs.