Abstract: This study aimed to investigate the regulatory effect and mechanism of thymopentin on the growth of subcutaneous hepatocellular carcinoma in mice. A subcutaneous tumor model of Hepa1-6 liver cancer in immunocompetent mice was constructed, with three randomly divided groups based on tumor volume: control group, low-dose thymopentin (TP5) group (10 mg/kg), and high-dose TP5 group (20 mg/kg), with 6 mice in each group. Drugs were administered, and the intervention effect of thymopentin on tumor growth was evaluated. Hepa1-6 cells were then cultured in vitro and treated with blank medium and TP5 of different concentrations (10, 100, 1000 ng/mL) for 72 hours. Cell viability was detected by sulforhodamine B (SRB) colorimetry. A subcutaneous tumor model of liver cancer LM3 in immunocompromised mice was constructed, with three randomly divided groups based on tumor volume: control group, TP5 group (20 mg/kg), and positive drug Sorafinib group (30 mg/kg). The intervention effect of thymopentin on the growth of subcutaneous tumors in immunocompromised mice was evaluated. Flow cytometry was used to analyze the changes in the proportion of T cells and myeloid-derived suppressor cells (MDSCs) in the tumor microenvironment 11 days after TP5 administration in the Hepa1-6 model. MDSCs were cultured in vitro and treated with TP5. The effect of TP5 on MDSCs was evaluated by detecting the levels of ROS, IL-6, and NO, which are effector molecules of MDSCs. The mouse subcutaneous liver cancer model was established again using C57BL/6N mice. After 10 days, they were randomly divided into four groups based on tumor volume: control group, low-dose TP5 group (10 mg/kg), high-dose TP5 group (20 mg/kg), and arginine-deficient TP5 group (15 mg/kg). Drugs were administered continuously for 11 days, and the intervention effect of arginine-deficient TP5 on tumor growth was evaluated based on tumor weight. Annexin-V staining was used to detect the impact of TP5 on T cell survival. The results showed that both low and high doses of TP5 inhibited the growth of subcutaneous liver cancer in immunocompetent mice (P < 0.05), yet TP5 had no direct inhibitory effect on the proliferation of tumor cells cultured in vitro. Besides, a high dose of TP5 could not inhibit the growth of subcutaneous liver cancer in immunocompromised mice. Furthermore, TP5 promoted the infiltration of CD4 and CD8 T cells but decreased MDSCs in the subcutaneous tumor microenvironment of immunocompetent mice. TP5 did not affect the levels of ROS, IL-6, and NO in MDSCs. Lastly, arginine-deficient TP5 could not inhibit the growth of subcutaneous liver cancer in immunocompetent mice. Accordingly, TP5 but not arginine-deficient TP5 promoted the increase in the proportion of viable CD4 and CD8 T cells cultured in vitro. These results suggest that TP5 may inhibit the growth of liver cancer by increasing T cell number in the liver cancer microenvironment.