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Objective: To explore the characteristics and functions of human umbilical cord-derived mesenchymal stem cell (hUC-MSC) after 10 passages (P10-hUC-MSC). Methods: Human umbilical cord was obtained from Xiamen Hongai Hospital (ethical lot number: HAXMMEC- 20201012-037-01), hUC-MSCs were isolated, collected, cultured and passaged, and P1-, P10- hUC-MSCs were collected. FCM was used to detect cell phenotypes. Senescence-associated β-galactosidase staining method and Annexin-V flow cytometry were adopted to detect end-stage cell senescence and apoptosis, respectively. Colchicine treatment was used to detect chromosomal stability. In vitro lipogenesis and osteogenesis induction assay was used to examine the ability of multidirectional differentiation of the cells. After co-culture with peripheral blood mononuclear cells (PBMC) at different ratios, the T-lymphocyte subsets and phenotypes were detected by FCM. Results: The phenotypes of P10-hUC-MSCs were similar to that of P1-hUC-MSCs, showing negative expression of CD45, CD34 and HLA-DR, but high positive expression rate (over 95%) of CD105 and CD90. Both groups of P1-hUC-MSCs and P10-hUC-MSCs showed positive β-galactosidase expression and early apoptotic characteristics, with no significant difference (P>0.05), and the cell chromosomes remained stable without transformation. Both P1- and P10-hUC-MSCs could be successfully induced and differentiated into adipocytes and osteoblasts in vitro without significant differences (P>0.05). When P10-hUC-MSCs and PBMCs were co-cultured in the ratio of 1:1, the proportion of CD4+/CD8+ T cells and CD4+ Treg cells, and PD-1 expression were all significantly upregulated ( all P<0.01). Conclusion: The long-term passaged P10-hUC-MSCs still maintain their biological characteristics and safety, and possess multi-differentiation and immunomodulatory ability, which provide preliminary experimental basis and guidance for maximizing its damage repair and prevention effects in radiotherapy. [ABSTRACT FROM AUTHOR]