In order to discuss the effect of alfalfa polysaccharides on mice immuno function, alfalfa polysaccharides were extracted from alfalfa, and the research of how lymphopoiesis were affected by alfalfa polysaccharides were set up to determine its immunoregulatory activity. MMT assay were adopted to detect the effect of alfalfa polysaccharides on the proliferation of normal Balb/c mice spleen lymphocytes in vitro as well as the cytotoxicity NK cell to K562 cell. The results indicated that, alfalfa polysaccharides could be activated to enhance the proliferation of mice spleen lymphocytes singly and stimulate T lymphocyte proliferation cooperated with ConA or stimulate B lymphocyte cooperated with LPS, which pointed that the organism’s cellular immunity and humoral immunity function could be regulated by alfalfa polysaccharides; alfalfa polysaccharides could significantly strengthen the killing activity of NK cell to K526 target cell, which indicated that alfalfa polysaccharides could markedly boost organism’s nonspecific immuno function. Consequently, alfalfa polysaccharides possess impressive immuno regulatory function.