Objective To study the inhibitory effect and mechanism of polysaccharide XP-10 from Aconiturn brachypodum Diels on lung metastasis of primary liver cancer, and to provide scientific basis for anti-tumor immunotherapy of Aconiturn brachypodum Diels polysaccharide and rational development of southern medicine plant resources. Methods A mouse model of H22 liver cancer with pulmonary metastasis was established and randomly divided into pathological model group, XP-10 low-dose group(250 mg/kg) and XP-10 high-dose group(500 mg/kg). After drug intervention, the number of intrapulmonary metastasis of liver cancer was counted; The viability of splenic lymphocytes in tumor bearing mice was detected by MTT assay. The expression levels of Th1 (CD4+ IFN-γ+ ), Th17 (CD4+ IL-17A+ ) and Treg cells (CD4+ Foxp3+ ) in spleen lymphocytes were detected by flow cytometry. Results XP-10 intervention significantly inhibited the number of pulmonary metastatic foci (P<0.01), and increased the proliferation of lymphocytes in tumor bearing mice(P<0.01); XP-10 low dose(250 mg/kg) intervention significantly decreased the proportion of CD4+ Foxp3+ regulatory T cells in tumor bearing mice(P<0.05), while XP-10 intervention had no significant effect on the proportion of Th17 cells and Th1 cells in lymphocytes of tumor bearing mice with lung metastasis from liver cancer(P>0.05). Conclusion The polysaccharide component XP-10 of Aconiturn brachypodum Diels can significantly inhibit the lung metastasis of primary liver cancer, and its inhibitory effect is partly achieved by downregulating Treg cells related to the negative regulation mechanism of immune response, promoting the proliferation of T lymphocytes, and enhancing the anti-tumor immune mechanism. This provides a certain modern scientific basis for the polysaccharide anti-tumor immunotherapy of traditional Chinese medicine Aconiturn brachypodum Diels in clinical practice.