As a traditional Chinese multiherbal formula, Yu-Ping-Feng (YPF) is frequently used to treat cold, flu and inflammation-associated diseases. We aimed to evaluate the immunostimulatory effects of polysaccharide isolated from YPF (YPF-PS) in vitro and in vivo. In in vitro experiment, macrophage cell proliferation, phagocytosis rate, cytokine and costimulatory molecule release, T lymphocyte proliferation, cell cycle distribution, CD4⁺ and CD8⁺ T cell percentages were determined. To investigate the in vivo effects of YPF-PS treatment, different doses YPF-PS were administered to chicken vaccinated against Newcastle disease. The immune organ index, lymphocyte proliferation, antibody titer, cell cycle distribution, and the cell percentage of CD4⁺ and CD8⁺ were assessed. In vitro results indicated that YPF-PS at 15.62μgmL⁻¹ could increase the LPS-induced macrophage cell proliferation and phagocytosis rate significantly. The levels of cytokine (nitric oxide, tumor necrosis factor-alpha, interleukin-1 beta, interleukin-6, and interferon beta) and costimulatory molecules (CD80 and CD86) were also considerably enhanced. Moreover, YPF-PS could significantly enhance T lymphocyte proliferation individually or synergistically with phytohemagglutinin. It promoted lymphocyte entry into S and G2/M phases and increased the percentages of CD4⁺ and CD8⁺ T cells effectively. In addition, in vivo experiments showed that YPF-PS could enhance serum HI antibody titer. The results about T lymphocyte proliferation, cell cycle distribution, CD4⁺ and CD8⁺ cell percentages in chickens were also confirmed. YPF-PS has efficacious immunomodulatory properties and could be used as a new potential immune stimulator for food and medical purposes.