Chemodynamic therapy (CDT) can effectively induce immunogenic cell death (ICD) in tumours and is thus a promising strategy for boosting the efficacy of immunotherapy. However, the mechanism by which CDT enhances ICD and lowers ICD efficiency is unknown and this restricts its clinical application. In this study, a second near-infrared (NIR-II) window irradiation-triggered hydrogen peroxide (HO) self-supplying nanocomposite ((CuSe-CaO)@LA) was constructed. The modified lauric acid was melted by the heat energy of the NIR-II irradiation, to expose the CaO nanoparticles, and they then reacted with water to produce HO and Ca. HO was then converted to hydroxyl radicals by the photothermal-enhanced CDT process of the CuSe nanocubes. Notably, the CDT and Ca overload was found to induce sequential damage to the mitochondria and endoplasmic reticulum (ER), which upregulated the PERK-mediated eIF2α phosphorylation pathway and caused subsequent ICD. NIR-II irradiation of the (CuSe-CaO)@LA also increased reactive oxygen species (ROS) formation and this was sufficient to increase dendritic cell maturation, attracting cytotoxic T lymphocytes, and suppressing tumour growth in vivo. Overall, we demonstrated that an enhanced CDT strategy under NIR-II exposure and HO self-supply can induce extensive ICD by inducing mitochondria-associated ER stress, which represents a highly effective and promising strategy for ICD amplification and tumour immunotherapy. STATEMENT OF SIGNIFICANCE: In this study, a second near-infrared window (NIR-II) irradiation-triggered and HO self-supplying nanocomposite (named (CuSe-CaO)@LA) was constructed and tested both in vitro and in vivo. These nanoparticles demonstrated promising antitumor activity as designed. Mechanistically, the nanoparticles could damage mitochondria and upregulate the PERK-mediated eIF2αphosphorylation pathway, further causing endoplasmic reticulum stress, and inducing immunogenic cell death through dendritic cell maturation and cytotoxic T lymphocyte recruitment augmented activity. This system represents a highly effective and promising strategy for enhancing tumor immunotherapy and provides new insights for future studies and design refinements.