Floral mushroom was a wood-rotting fungus. A homogeneous polysaccharide, named FMP2-1, was extracted from floral mushroom grown in the Huangshan Mountain and further purified by DEAE-cellulose and Sephacryl S-300 column chromatography. Monosaccharide analysis showed that FMP2-1 was glucan. The structural features of FMP2-1 were carried out using methylation analysis, infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (NMR). The results revealed that FMP2-1 had a backbone chain consisting of a (1→4)-α-D-Glcp and (1→6)-β-D-Glcp with the branched chain T-β-D-Glcp attached to the O-6 position of (1→4)-α-D-Glcp. Immunostimulatory activity was measured via murine macrophage cell line RAW264.7 cells in vitro. The FMP2-1 exhibited significant in macrophage proliferation, NO production and phagocytic activity with optimum dose of 200 μg/mL, 100 μg/mL and 200 μg/mL, respectively. Therefore, the deletion of branching and reduction of average molecular weight caused by partial acid hydrolysis was demonstrated to weaken the immunomodulatory activity of FMP2-1. Results suggested FMP2-1 could be a potential immunomodulatory agent for food industry.