[Objectives]The paper aimed to investigate the immunological enhancement of acidic Alhagi sparsifolia Shap polysaccharide(aASP)on mouse splenic lymphocytes. [Methods]aASP was extracted by alcohol precipitation, separated and purified by column chromatography, and the contents of sugar, uronic acid, and protein were detected using phenol sulfuric acid method, sulfuric acid-carbazole method, and BCA microplate reader. The polysaccharide properties were identified by infrared spectroscopy. MTT assay was used to determine the safe concentration of aASP on mouse splenic lymphocytes, and the proliferative effects of aASP alone and synergistic lipopolysaccharide(LPS)and phytoheamagglutinin(PHA)on splenic lymphocytes were detected at three concentrations of 500, 125 and 31.25 μg·mL-1 within the safe concentration range; the effect of aASP on lymphocyte differentiation was detected by flow cytometry; the effect of aASP on lymphocyte cytokine secretion was detected by ELISA. [Results]The contents of polysaccharide, uronic acid, and protein in aASP extracts were(81.8±1.2)%, (17.34±0.75)%, and(0.54±0.08)%, respectively. Infrared spectra showed that aASP had a strong absorption peak at 1 746.53 cm-1, and combined with the content of uronic acid, aASP could be identified as acidic polysaccharide. High, medium, and low concentrations of aASP could promote the proliferation of splenic lymphocytes, and aASP synergistic with LPS and PHA could significantly promote the proliferation of B cells and T lymphocytes(P<0.05). The results of flow cytometry and ELISA showed that aASP was able to promote the differentiation of CD4+ and CD8a+ T cells and promote the secretion of IFN-γ, TNF-α, IL-6, and IL-4 in splenic lymphocytes. [Conclusions]aASP can boost the proliferation of splenic lymphocytes in mice, induce Th1(helper T lymphocyte 1)and Th2(helper T lymphocyte 2)mixed immune responses in splenic T lymphocytes, and the immune cells are biased towards CD4+ T cell differentiation. [ABSTRACT FROM AUTHOR]