In this study, the structural characterization and anti-inflammation effect of dilute alkali-soluble polysaccharides from purple sweet potato were investigated. Three fractions (F-1, F-2 and F-3) were obtained by purifying crude polysaccharides on DEAE-52 cellulose column. The main fraction (F-1) was further purified on Sephadex G-200 column to afford purified alkali-soluble sweet potato polysaccharide (ASPP). The chemical structure of ASPP was analyzed by gas chromatography, Fourier transform infrared spectroscopy, methylation analysis and nuclear magnetic resonance spectroscopy. Monosaccharide compositional analysis showed ASPP was composed of rhamnose, arabinose, xylose, mannose and glucose in the molar ratio of 2.8:1.9:1.0:7.6:53.3. Moreover, the backbone of ASPP was composed of 1,4-linked Glcp with side chains attached to the O-6 position. The anti-inflammation effect of ASPP was further investigated by in vitro and in vivo experiments. Results showed ASPP could inhibit the levels of nitric oxide, interleukin (IL)-6, IL-1β and TNF-α but increase the production of IL-10 in lipopolysaccharide (LPS)-treated RAW 264.7 macrophage cells. In addition ASPP could reduce the secretion of IL-6, IL-1β and TNF-α in LPS-treated mice. Our results suggest ASPP can be developed as a novel anti-inflammation agent.