A water-soluble polysaccharide named DI was extracted from the fruiting bodies of gastroid mushroom Dictyophora indusiata with boiling water. The chemical and physical characteristics of DI were investigated by a combination of chemical and instrumental analysis methods. The immunomodulatory activities on RAW 264.7 macrophage of DI in vitro were also studied. The results showed that DI is a β-(1→3)-glucan with side branches of β-(1→6)-glucosyl units, and it has triple-helical structure. DI has no toxic effect on cells, but can promote macrophage multiplication. DI significantly affects the immune function by promoting the production of nitric oxide and cytokines, such as tumor necrosis factor-α, interleukin-1, -6, and -12, showing an obvious dose-effect relationship. This work extends the application scope of the polysaccharide from D. indusiata in the biomedical field.