According to the world epidemic report, the mortality of patients with severe coronavirus disease 2019 (COVID-19) is high. Diabetic patients are more susceptible to COVID-19. Since the mortality of COVID-19 patients with diabetes is on the top of list, hyperglycemia is considered an independent risk factor for severe COVID-19. Up to now, there is few effective treatment for severe patients infected with 2019 novel coronavirus (2019-nCoV). Clinical studies observed that cytokine storms existed in patients with severe COVID-19. Sustained high levels of cytokines cause diffuse damage to pulmonary capillary endothelial cells and alveolar epithelial cells, resulting in acute respiratory distress syndrome (ARDS). ARDS is the main cause of death in COVID-19 patients. Host-directed therapy (HDT) is an emerging therapeutic method in the field of anti-infection, which can activate the self-protective immune response, suppress excessive inflammatory response, and be used to assist the treatment of traditional drugs to shorten the course of disease. Metformin has been shown to be effective in HDT and can assist in the treatment of the viral and bacterial infectious disease. This paper discusses the rationality and potential therapeutic mechanism of metformin in the treatment of severe COVID-19. It was speculated that the use of metformin for controlling blood glucose in severe COVID-19 patients with diabetes may prevent or inhibit the occurrence of ARDS, thereby reducing the mortality of COVID-19 patients. The possible mechanism is that metformin could inhibit cytokine storm via suppressing interleukin-6 (IL-6) signaling, prevent the process of lung fibrosis, suppress endocytosis, thereby elevating angiotensin converting enzyme 2 (ACE2) expression.