Sanghuang mycelial polysaccharides (SVMPs) were studied using poplar sanghuang as raw material (sanghuangporus vanninii, SV). The bioactivity of SVMPs was evaluated through in vitro antioxidant and cellular immunity activity assays, and the acute oral toxicity and anti-fatigue ability of SVMPs in mice were determined using mice as an experimental animal model. The results showed that SVMPs have a good scavenging effect on 1,1-diphenyl-2-picrylhydrazyl (DPPH), hydroxyl, and 2,2′-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS+) radicals, with a slightly weaker reducing power. SVMPs promote, to some extent, the secretion of cytokines Tumor Necrosis Factor-α (TNF-α), Interleukin-1β (IL-1β), and Interleukin-6 (IL-6), and the release of NO from mouse macrophage RAW264.7. In mouse experiments, the mouse tolerance of SVMPs was more greater than 18.2 g/kg BW, which was 2724 times the daily dosage of humans, and it was a non-toxic substance; the high dose group of SVMPs could significantly prolong the swimming time of weight-bearing mice. Thus, the results of this study clearly demonstrate the biological activity of SVMPs and provide a valuable basis for the potential application of SVMPs as antioxidants, immunomodulators, and functional foods.