Background The COVID-19 cases increased very fast in the last two months. The mortality among critically ill patients, especially the elder ones, was relatively high. Considering that most of the dead patients were caused by severe inflammation response, it is very urgent to develop effective therapeutic agents and strategies for these patients. The human umbilical cord mesenchymal stem cells (hUCMSCs) have shown very good capability to modulate immune response and repair the injured tissue with good safety. Case Presentation Here, we reported the treatment process and clinical outcome of a 65-year-old female critically ill COVID-19 patient infected with 2019-nCoV (now called SARS-CoV-2). The significant clinical outcome and well tolerance was observed by the adoptive transfer of allogenic hUCMSCs. Conclusions Our results suggested that the adoptive transfer therapy of hUCMSCs might be an ideal choice to be used or combined with other immune modulating agents to treat the critically ill COVID-19 patients.