Stem cell therapy, as a new type of treatment for certain refractory diseases, has important clinical value in the treatment of tissue repair, autoimmune diseases and degenerative diseases. In this study, nude mice were used as animal models to evaluate the safety of human umbilical cord mesenchymal stem cells through experiments such as subcutaneous tumor formation, clone formation and telomerase activity in nude mice, and provide a reference for the safety of stem cells for clinical treatment. The results of the tumorigenicity test in nude mice showed that the number of tumors in the negative control and in the experiment were 0, and the number of tumors in the positive control was 12. The results of the clone formation test showed that the negative control group and the experimental group had no clonal formation, while the positive control group had clone formation. The telomerase activity test results showed that the negative control group had a relative telomerase activity of 0.01, the positive control group had a relative telomerase activity of 4.33, and the relative activity of granzyme for the experimental group which had P6 generation cell was 1.70. Therefore, the evaluation results of this study showed that human umbilical cord mesenchymal stem cells were not tumorigenic in nude mice, and the clone formation test result was negative, the telomerase activity test result was normal and had good genetic stability. Human umbilical cord mesenchymal stem cells showed good safety in this experiment and have good clinical application value.