Abstract: Polysaccharide was isolated and purified from Chinese medicinal herb Ramulus mori (Sangzhi), and determination method for Ramulus mori polysaccharide (RMPS) was developed in this study. Carbon clearance test and dinitrylfluorobenzene-induced delayed type hypersensitivity test (DTH) were used to evaluate effects of RMPS on immune responses of mice treated with dexamethasone. The results indicated that using modified phenol-sulfuric acid method, the content of polysaccharide was 5.5% in Ramulus mori, and 56.6% in RMPS, which exceeded 70% in RMPS after purifying for a few times. The RMPS can improved phagocytic indexα, strengthened phagocytosing function of reticulate endothelial cell and the intensity of delayed type hypersensitivity (DTH) reaction, and enhanced the activity of T cells of the mice treated with dexamethasone, significantly.