To develop a safe and novel immunoadjuvant to enhance the immunity and resistance of animals against E.coli infection. Methods An 88-base immunostimulatory oligodeoxynuleotide containing eleven CpG motifs (CpG ODN)was synthesized and amplified by PCR. The chitosan nanoparticle (CNP) was prepared by ion linking method to entrap the CpG ODN that significantly promotes the proliferation of lymphocytes of pig in vitro. Then the CpG- CNP was inoculated into 21-day old Kunming mice, which were orally challenged with virulent K88/K99 E. Coli 35 days after inoculation. Blood was collected from the tail vein of mice on days 0, 7, 14, 21, 28, 35, 42, and 49 after inoculation to detect the changes and content of immunoglobulins, cytokines and immune cells by ELISA, such as IgG, IgA, IgM, IL-2, IL-4, and IL-6. Results The CpG provoked remarkable proliferation of lymphocytes of pig in vitro in comparison with that of control group (P＜0.05). The inoculation with CpG-CNP significantly raised the content of IgG, IgM, and IgA in the sera of immunized mice (P＜0.05). The levels of IL-2, IL-4, and IL-6 in the mice significantly increased in comparison with those in controls (P＜0.05), so was the number of white blood cells and lymphocytes in immunized mice. The humoral and cellular immunities were significantly enhanced in immunized mice, which resisted the infection of E. coli and survived, while the control mice manifested evident symptoms and lesions of infection. Conclusions CpG-CNP can significantly promote cellular and humoral immunity and resistance of mice against E. coli infection, and can be utilized as an effective adjuvant to improve the immunoprotection and resistance of porcine against infectious disease.