Astragalus polysaccharides (APS), an extract from a kind of Chinese traditional herb Astragalus membranaceus, was proved to have strong immunoregulatory properties. In this study, APS was employed as an adjuvant of Hepatitis B virus (HBV) DNA vaccine (pcDS2) and its' effects on immune system of mice were investigated. Our data demonstrated that APS as an adjuvant could increase the HBsAg-specific antibody level as well as the proliferating activity of T cells. APS also could induce CD4(+) T cells to produce IL-4, IL-2 and IFN-γ and enhance IFN-γ expression of CD8(+) T cells. Moreover, APS could induce the robust activity of the cytotoxic lymphocytes (CTL). Additionally, APS could stimulate the dendritic cells (DC) maturation which is characterized by up-regulation of MHC I/II, CD40, CD80 and CD86, and decreased the frequency of the regulatory T cells (nTreg). Collectively, these findings suggest that APS is a potent adjuvant for the hepatitis B DNA vaccine and can enhance the immune responses of HBV DNA vaccine via promoting DC maturation and inhibit the Treg frequency.