Abstract: In this study,polysaccharides (RF-S) was isolated from the Ramaria flaccida (Fr.) Quél,which was used as the research object of study. Its structure was studied by infra-red spectrum,and the immunomodulatory activity of RF-S was studied in vitro by using cytological techniques and using macrophages and lymphocytes as target cells. The results showed that the polysaccharide structure of RF-S was typical without any other structure,and it was a kind of pure polysaccharide. And the monosaccharide in its polysaccharides existed in the form of pyranose. In addition,with the increase of RF-S concentration,the proliferation of macrophages,T lymphocytes and B lymphocytes could be extremely significantly promoted (p<0.01),and showed a dose-dependent relationship. Meanwhile,when the mass concentration of RF-S was 5 μg/mL,it could extremely significantly promote phagocytosis of neutral red and fluorescent microspheres in macrophage,and could extremely significantly stimulate macrophages to secrete IL-2,IL-10,TNF alpha and release NO (p<0.01). It also had a significant effect on stimulating the secretion of IL-6 in macrophages (p<0.05). In conclusion,polysaccharides from the Ramaria flaccida (Fr.) Quél had good immunomodulatory activity.