Irradiation-induced skin ulcers can be resultant from nuclear accident or reaction to radiation therapy of tumor and is intractable for healing. Human umbilical cord mesenchymal stem cells (hUC-MSCs) have been considered to be the potential therapeutic tools for tissue regeneration. However, the underlying mechanisms are still not well understood. This study aims to investigate the effects of hUC-MSCs on irradiation-induced skin ulcers healing and the related mechanisms. The ulcers were induced by irradiating the skin of adult SD rats. The ulcers of SD rats were treated with vehicle or hUC-MSCs donated from mother giving birth. The ulcer healing was measured by imaging the healing rate and the H&E staining. CD31 and VEGF expression was measured with immunohistochemistry assay. iTRAQ proteomics analysis was used to analyze the signaling pathway. The results showed that hUC-MSCs improved healing of irradiation-induced skin ulcers in vivo using a rat model of skin ulcer. Transplantation of hUC-MSCs promoted keratin generation and keratinocytes proliferation of ulcer areas. Furthermore, the results demonstrated that hUC-MSCs increased expression of CD31 and VEGF in ulcers and promoted neovascularization. iTRAQ proteomics analysis results indicated that PI3K/Akt signaling pathway involved in hUC-MSCs-mediated repairing of irradiation-induced skin ulcer. In conclusion, human umbilical cord mesenchymal stem cells promoted neovascularization and re-epithelization, and improved healing of irradiation-induced skin ulcers. This healing improvement may be conducted through activating the PI3K/Akt signaling pathway, however, which needs to be proven by the further investigations.