Based upon immune cytokine expressions in mice serum and lung tissues, the expression of immune cells in spleens and the regulation of related proteins in PI3K/AKT signalling pathway and EMT signalling pathways, the inhibition mechanism of BC pulmonary metastasis was summarized as follows: cGAMP administration could activate STING immune pathway, restore IRF3 phosphorylation and induce IFN-β production, which activated innate immunity and adaptive immunity response, promoted CD8+ T cell proliferation and produced IFN-γ. The gene discussed is IFNG; the disease is breast cancer.