PURPOSE: To explore the effect of calcium peroxide nanoparticles (CaO NPs) combined with programmed cell death protein 1 (PD-1) inhibitors in the treatment of liver cancer and its related mechanism. METHODS: Hepa1-6 cells were cultured to construct the Hepa1-6 mouse liver cancer model. In vivo mechanism study, a unilateral tumor model was established. Eighteen tumor-bearing mice were randomly divided into the control group (intra-tumoral injection of PBS solution) and the experimental group (intra-tumoral injection of CaO NPs). A hypoxic probe, pH probe, and micro-CT were used to evaluate the effect of CaO NPs on improving hypoxia, neutralizing acidity, and inducing calcium overload within the tumor. To study the effect of CaO NPs combined with PD-1 inhibitors on proximal and distal tumors, the bilateral tumor model was established. Forty tumor-bearing mice were randomly divided into the control group (intra-tumoral/intra-peritoneal injection of PBS solution), CaO NPs group (intra-tumoral injection of CaO NPs), PD-1 group (intra-peritoneal injection of PD-1 inhibitor), and the combination group (intra-tumoral injection of CaO NPs and intra-peritoneal injection of PD-1 inhibitors). The administered side was recorded as the proximal tumor. Tumor volume and body weight were measured every 2 days after treatment. On day 8, serum and tumor samples were collected. The immune factors in serum (Interferon-γ (IFN-γ), Tumour necrosis factor-α (TNF-α), Interleukin-2 (IL-2), and Interleukin-10 (IL-10)) and tumor tissue (IFN-γ and TNF-α) were detected by ELISA. H&E staining was used to detect tumor necrosis. Immunohistochemical staining was used to detect the amount of CD4 and CD8 T cells within the tumor. By analyzing the tumor volume, pathological indexes, and immune-related indexes, the effects of CaO NPs combined with PD-1 inhibitors on proximal and distal tumors were evaluated, and they mediated immunomodulatory effects (including local and systemic immunity), and their effects on tumor burden were studied. In addition, a unilateral tumor model was established to study the effect of CaO NPs combined with PD-1 inhibitors on survival time. RESULTS: The results of in vivo mechanism study showed that CaO NPs can improve hypoxia, neutralize acidity, and induce calcium overload within tumors. The results of the study on the effect of CaO NPs combined with PD-1 inhibitor on proximal and distal tumors showed that, compared with the other three groups, the bilateral tumor burden of the combination group was significantly reduced, the intra-tumoral infiltration of CD8 and CD4 T cells were significantly increased, the secretion of anti-tumor immune factors in tumor and serum was increased, and the secretion of pro-tumor immune factors was decreased. Mice in the combination group showed the longest survival compared with the other groups. CONCLUSION: CaO NPs can improve hypoxia, neutralize acidity, and induce calcium overload within tumors, so as to reduce tumor burden and realize an immunosuppressive tumor transformation to a hot tumor, and play a synergistic role with PD-1 inhibitors in anti-liver cancer.