The purpose of the current study was to characterize the polysaccharides isolated from the moxa (Artemisia argyi) leaf and investigate their bioactivities. The molecular weights of the purified polysaccharides were at a range from 6,000 to 10,000 Da and consisted mostly of carbohydrates (>90%), mainly the monosaccharide of xylose, arabinose, mannose, and glucose. The isolated polysaccharides were further fractionated to form 4 subfractions and all of the subfractions exhibited high antioxidant capability on both hydroxide ($IC_{50}$ ranging from 8 to 18 ${mu}g$/mL) and superoxide anion radicals ($IC_{50}$ ranging from 10 to 30 ${mu}g$/mL). The moxa leaf polysaccharides were also capable of enhancing the Concanavallin A (ConA)-induced T cell proliferation, but not the lipopolysaccharides (LPS)-induced B cell proliferation in the in vitro immunological tests. Furthermore, the polysaccharides also strongly facilitate ConA-induced secretion of interferon-${gamma}$ (IFN-${gamma}$) and interleukin-2 (IL-2) in a dose-dependent manner. The results suggested that the moxa leaf polysaccharides may have potential applications as antioxidants and immune enhancers.