Selenium-enriched Hypsizygus marmoreus mycelium polysaccharide (Se-HMP) and H. marmoreus mycelium polysaccharide (HMP) were prepared from the powder of H. marmoreus before and after selenium-enriched fermentation， respectively， and then investigated their physical and chemical properties， microscopic morphology， chemical bonding mode and immune activity. The properties were conducted by uronic acid content determination， Congo red test， Fourier transform infrared spectroscopy and electron microscope. The immune activity was evaluated by means of proliferative activity， phagocytic activity and cytokine content using mouse macrophage RAW264.7 model. The results indicated that the yield of crude polysaccharide from selenium-enriched H. marmoreus was (22.90±1.62)%， which was 1.99 times that of H. marmoreus without selenium-enrichment. The content of uronic acid in Se-HMP was (15.60±1.46)%， which was significantly higher than that in HMP (P<0.05). Both Se-HMP and HMP have no triple helix structure， but their micromorphology is quite different. Polysaccharides in SE-HMP are bound with selenium in the form of Se=O bond and O-SE-O bond. In the range of 0.05-1.00 mg/mL， Se-HMP increased the proliferative activity of RAW264.7 cells and significantly promoted the secretion of NO and TNF-α (P<0.05) in comparison with HMP， but their ability to up-regulate IL-6 level was similar. In addition， both Se-HMP in low mass concentration (≤0.25 mg/mL) and HMP in high mass concentration (≥0.50 mg/mL) can promote phagocytosis of macrophages. Therefore， Se-HMP has higher immune activity in comparison with HMP， which may be due to the organic binding of polysaccharide and selenium and the higher content of uronic acid in Se-HMP.