Triple-negative breast cancer (TNBC), which accounts for 10%-20% of breast cancer cases, is characterized by a higher metastasis rate, higher recurrence risk, and worse prognosis. Traditional treatments such as chemotherapy, surgery, and radiotherapy have limited therapeutic effects. Although immune checkpoint blockade (ICB) therapy represented by anti-programmed death 1 (aPD-1) antibody has made further progress in treating TNBC, its therapeutic effect is still not optimistic. Ataxia telangiectasia mutated (ATM) is a critical factor in the DNA damage response (DDR) pathway, which is associated with the development of tumors. Recent studies have found that it can regulate the tumor immune microenvironment, affecting ICB responsiveness. Inhibition of ATM could enhance ICB therapy by promoting mitochondrial DNA cytoplasmic leakage and activating the innate immune signaling pathway. To explore the effect of ATM siRNA(siATM) on the ICB responsiveness of TNBC, we designed and synthesized nanoparticles using 1,2-dioleoyl-glycero-3-phosphatidylcholine (DOPC) liposomes to deliver siATM. In vitro and in vivo experiments demonstrated that DOPC/siATM could enhance the ability of siRNA to enter tumor cells and effectively inhibit the expression of ATM protein. Our study found that nanoparticles carrying siATM could activate cytotoxic T lymphocytes and regulate the immunosuppressive tumor microenvironment (ITM) by activating the cGAS-STING pathway. Its combination with aPD-1 may be a potential way to improve the efficacy of TNBC.