Objective： To study the immunomodulation and colon health effect of polysaccharide fractions from Dendrobium officinale in mice. Methods： The polysaccharides isolated by alcohol precipitation of Dendrobium officinale and have different molecular weights. The immunosuppressed mouse model was established by intraperitoneal injection of cyclophosphamide. To evaluate the immunomodulation effect of polysaccharide fractions from Dendrobium officinale， the changes of thymus index， serum and spleen cytokine secretion were determined before and after gavage of the polysaccharides. The levels of water content， pH value and short-chain fatty acid content of feces in cyclophosphamide model group and polysaccharide fractions groups were also measured to evaluate the effect of polysaccharide fractions on colon health. Results： Compared with the cyclophosphamide model group， DOP group and F50S group can significantly improve the thymus index of mice； promote the secretion of IFN-γ， TNF-α， IL-6； inhibit the secretion of IgE； regulate the balance of Th1/Th2 cell responses shift to Th1 cell responses then enhance the immune function of mice. In addition， they can reduce the pH of feces， increase fecal water content and SCFAs levels which could promote colon health. Conclusion： polysaccharide fractions from Dendrobium officinale can effectively enhance the immune regulation of immunosuppressed mice and improve their colon health， however， DOP is more effective than each polysaccharide fraction. In addition， the effect of F50S fraction with the smallest Mw is better than other fractions.