The bursa of Fabricius (BF) is the central humoral immune organ unique to birds. The bursal peptide BPs (BP-I, BP-II, BP-III, BP-IV) were isolated and identified from BF. In this study, the immunoadjuvant activities of BP-(I-IV) were examined in mice immunized with H9N2 avian influenza virus (AIV) vaccines. The results suggested that BP-I effectively enhanced cell-mediated immune responses, increased the secretion of Th1 (gamma interferon [IFN-γ]) and Th2 (IL-4)-type cytokines, and induced a better cytotoxic T-lymphocyte (CTL) response to the H9N2 virus. BP-II mainly elevated specific-antibody production, especially neutralizing antibodies, and increased Th1- and Th2-type cytokines secretion. BP-III had no significant effect on antibody production and cell-mediated immune responses compared to the control group. A strong immune response at both the humoral and cellular levels was induced by BP-IV. Furthermore, a virus challenge experiment and HE staining revealed that BP-I and BP-II promoted removal of the virus and conferred protection in mice lungs. BP-IV significantly reduced viral titers and histopathological changes and contributed to protection against H9N2 AIV challenge in mice lungs. This study further researched the immunoadjuvant activities of BP-(I-IV) peptides, providing a novel insight on immunoadjuvant for vaccine design.