Grifola frondosa, a species of Basidiomycotina, is an edible medicinal mushroom with a large fruiting body characterized by overlapping caps. The β-glucan is the major biologically active component in G. frondosa polysaccharide (GFP) or D-fraction, which has been studied extensively for nearly 30 years. GFP was approved as an adjunctive therapeutic drug in China for treating cancers in 2010. In this article, based on the search results of Chinese VIP, CNKI, and Wanfang databases, 105 independent animal studies were summarized. The chemical structure, the antitumor, immunomodulatory, anti-diabetic, anti-hyperlipidemia, and antiviral activities and molecular mechanisms of GFP are reviewed and discussed.