AIM: To assess the therapeutic effect of lienal polypeptide injection (LPI) on bone marrow suppression and poor immunity in Kunming (KM) mice after the intervention of chemotherapy drug carboplatin (CBP), as well as its potential mechanisms. METHODS: KM female mice were randomly divided into control group, model group, low-dose lienal polypeptide,and high-dose lienal polypeptide treatment groups. On day 1, mice in the treatment group and model group were subjected to intraperitoneal single injection of carboplatin (70 mg/kg), to induce chemotherapy-induced bone marrow suppression in mice,while the control group was intervened with normal saline. From Day 2 to Day 16, the treatment groups received daily intraperitoneal injections of lienal polypeptide (60 mg·kg-1·d-1), while the control and model groups received saline injections. At the end of the drug intervention, the number of platelets, leukocytes and erythrocytes in the peripheral blood of mice were measured. Mice body weight, organ indices of mice thymus and spleen were obtained; Flow cytometry was used to obtain changes in CD4+ T cell subsets, CD8+ T cell subsets, and CD4+CD25+Foxp3+ regulatory T cell (Treg) subsets in the thymus and spleen of KM female mice. RESULTS: Compared to the control group, mice in the model group exhibited significant reductions in white blood cell and platelet counts on Day 16, decreased organ indices of thymus and spleen, and reduced proportions of thymic and splenic Treg lymphocyte subsets.  Following high-dose lienal polypeptide intervention, white blood cell and platelet counts significantly recovered, and proportions of thymic and splenic Treg lymphocyte subsets were restored (P<0.05). CONCLUSION: Lienal polypeptide injection exhibits a therapeutic effect on bone marrow suppression and immune deficiency in KM mice post chemotherapy, as evidenced by the restoration of blood parameters and Treg lymphocyte subsets in the thymus and spleen.