Interleukin 2 (IL-2) is widely used as an active immunotherapeutic agent in clinical metastatic cancers. However, its therapeutic concentrations do not last long due to its short half-life. Thus, only a transient proliferation of the anti-cancer CD8(+) T cells can be achieved, resulting in poor efficacy. Therefore, the aim of this work was to create a system that promotes CD8(+) T cell proliferation at the tumor site using IL-2 persistently present and activates an anti-cancer immune response. This goal was achieved by the design of the IL-2-loaded polypeptide nanoparticles (P-IL-2) where methoxy poly(ethylene glycol) block poly-[(N-2-hydroxyethyl)-aspartamide] phenylboronic acid was used to encapsulate IL-2 through boron-nitrogen coordination with poly((L)-lysine). P-IL-2 significantly prolonged the circulation time of IL-2 and achieved a selective drug release at the tumor site in the presence of high levels of reactive oxygen species, thus activating an anti-cancer immune response and exerting a better anti-cancer effect. The half-life of P-IL-2 was 3.15-fold higher than that of IL-2, and the quantity of CD8(+) T cells after using P-IL-2 was 1.89-fold higher than that after using IL-2. In addition, the combination of P-IL-2 and anti-CTLA-4 monoclonal antibody resulted in an enhanced immune activation. Hence, this work provides a new approach to improve the efficacy of IL-2 in anti-cancer immunotherapy.