AbstractNanomedicine can regulate the balance between cytotoxic T lymphocytes (CTLs) and suppressive regulatory T lymphocytes (Tregs), which however has been rarely exploited for cancer immunotherapy. We report a charge‐reversal polymer nano‐modulator (SPDMCN) activated by tumor microenvironment (TME) for photodynamic immunotherapy of cancer. SPDMCN is constructed by conjugating an immunomodulator (demethylcantharidin, DMC) to the side chains of a photodynamic polymer via an acid‐liable linker. The negative charge of SPDMCN ensures its high stability in blood circulation and ideal tumor accumulation; exposure to acidic TME reverses its surface charge to positive, enhancing tumor penetration and locally releasing DMC. Upon near‐infrared photoirradiation, SPDMCN generates singlet oxygen to ablate tumors and promote maturation of dendritic cells. Released DMC inhibits protein phosphatase 2 (PP2A) activity and decreases Tregs differentiation. Such combinational action induces a sharp increase in CTL/Treg ratio in TME and effectively inhibits both primary and distant tumors in living mice.