To observe the safety of human umbilical cord mesenchymal stem cells (hUC-MSCs) injected into rats for many times and its effect on rat's liver, so as to provide support for the safety hUC-MSCs in clinical application.Methods Fifty SPF healthy SD rats were randomly divided into negative control group, vehicle control group, high dose, medium dose and low dose groups of hUC-MSCs, with ten rats in each group.The drugs were injected into the caudal vein 4 times, once a week.At 24 hours after the last administration, blood samples were collected for the detection of blood biochemistry, serum cytokine IFN-γ、IL-17、TNF-α and T lymphocyte subsets.The liver weight, organ body ratio index and histopathological changes were observed.Total RNA was extracted from rat liver tissue, and the expression of Act1, IL-17A and Hsp90aa1 genes was analyzed by qRT-PCR.Results After multiple intravenous injections of 1×107, 5.0×106, 2.5×106 cells/kg hUC-MSCs, there were no significant differences in blood biochemistry,serum cytokines IFN-γ, IL-17, TNF-α, T lymphocyte subsets, liver weight, organ body index and histopathological changes between negative control group and solvent control group (P＞0.05).hUC-MSCs could significantly reduce the expression of IL-17A mRNA in liver tissue, but had no significant effect on the expression of Act1 and hsp90aa1 mRNA.Conclusion hUC-MSCs are safe to be injected intravenously at doses of 1×107、5.0×106、2.5×106 cells/kg, and can reduce the level of IL-17A