In the field of immunotherapy, immune vaccinations have received more and more attention for disease prevention and treatment. In immune vaccination, efficient vaccine adjuvants are necessary due to the weak immunogenicity of vaccines. Some traditional vaccine adjuvants have been widely used but have shown obvious limitations such as poor biosafety. Therefore, researchers make a great effort to develop more functional novel immune adjuvants such as chitosan-based immune adjuvants. However, chitosan is poorly water soluble, which greatly limits its application as immune adjuvants, regardless of its good biocompatibility, biodegradability, and other biological activities. In this work, we prepared a water-soluble chitosan derivative phosphorylated chitosan (PCS) and evaluated its potential as a novel immune adjuvant. PCS was found to be pH sensitive: specifically, it was water soluble at pH < 7.0 but began to gel at pH >7.0. By virtue of this, neutral PCS aqueous solutions containing ovalbumin (OVA) antigen was intramuscularly injected into test mice, which would transform to an OVA-containing gel network for OVA immunization. The results showed that the use of 30 mg/mL PCS-based hydrogel as vaccine delivery system contributed to significantly higher level of antigen-specific immune responses, including higher level of antigen-specific IgG antibodies, IFN-γ and IL-4 cytokines secretion by splenocytes, as well as memory CD4 and CD8 T cells. In vivo imaging and immunohistochemistry assays suggest that the improved immunization efficacy may be attributed to the controlled release of antigen from injection site by PCS gel network, and then prolonged antigen stimuli to the immune system. From the results, PCS could be developed as a promising vaccine delivery system for immunotherapy.