Powerful adjuvants to augment vaccine efficacy with a less immunogenic vaccine system are in great demand. In this study, a novel squalene‐based cationic poly(amino acid) adjuvant (CASq) that elicits both cellular (Th1) and humoral (Th2) immune responses is developed. CASq is demonstrated to promote cellular uptake of viral antigen and stimulate macrophages, leading to active production of interleukin‐12. Furthermore, co‐administration of inactivated pdm H1N1 vaccine with CASq significantly increases the generation of antigen‐specific antibodies and T cell immune responses in mice, as well as resulting in complete prevention of disease symptoms and protection against lethal infection. A cationic amphiphilic graft poly(amino acid)‐based oil‐in‐water emulsion adjuvant, containing squalene (CASq) enhanced Ag cellular uptake and improved immune‐stimulation activity of vaccines against the influenza virus, by eliciting both humoral and cellular immunity. CASq is a novel adjuvant for highly specific and powerful immune responses, showing potential in future infectious disease prevention.