Objective: To compare the therapeutic effects and safety of dexmedetomidine and midazolam on patients with severe coronavirus disease 2019 (COVID-19) who received non-invasive ventilation., Methods: Patients with COVID-19 who needed non-invasive ventilation in one critical care medicine ward of Wuhan Jinyintan Hospital during the team support period from the department of critical care medicine of Renmin Hospital of Wuhan University from January 23rd to February 15th in 2020 were investigated retrospectively. Ramsay score, mean arterial pressure (MAP), heart rate (HR), respiratory rate (RR), arterial oxygen partial pressure (PaO 2 ) before sedation and at 1, 12, 24 hours after sedation, sleep time were collected, and the side effects such as excessive sedation, fall of tongue, abdominal distension, aspiration, bradycardia, escalation to invasive mechanical ventilation during 24 hours were also collected. According to different sedative drugs, patients were divided into the control group (without sedative drugs), dexmedetomidine group and midazolam group. The changes of indicators among the three groups were compared., Results: Fourteen patients were injected with dexmedetomidine (loading dose of 1 μg/kg for 10 minutes, maintained at 0.2-0.7 μg×kg -1 ×h -1 ); 9 patients were injected with midazolam (loading dose of 0.05 mg/kg for 2 minutes, maintained at 0.02-0.10 mg×kg -1 ×h -1 ); 12 patients didn't use sedative drugs due to limitations of previous hospital or patients' rejection. In dexmedetomidine group and midazolam group, the Ramsay score was maintained at 2-3 points after sedation, which were higher than those of control group at different time points after sedation, and there was no significant difference between dexmedetomidine group and midazolam group. MAP of dexmedetomidine group and midazolam group decreased gradually after sedation. MAP after 1-hour sedation was significantly lower than that before sedation, and MAP after 24 hours sedation was significantly lower than that in the control group [mmHg (1 mmHg = 0.133 kPa): 109.7±11.5, 107.1±12.3 vs. 121.1±13.3, both P < 0.05]. HR decreased gradually after sedation treatment, which was significantly lower after 12 hours of sedation than that before sedation, and HR in dexmedetomidine group was significantly lower than that in control group after 12 hours of sedation (bpm: 84.0±13.9 vs. 92.8±15.4 at 12 hours; 81.0±16.7 vs 92.6±12.7 at 24 hours, both P < 0.05). PaO 2 increased and RR decreased in all three groups after ventilation. PaO 2 in dexmedetomidine group and midazolam group were significantly higher than that in the control group after 12 hours of sedation [cmH 2 O (1 cmH 2 O = 0.098 kPa): 79.0±6.5, 79.0±8.9 vs. 70.0±7.8, both P < 0.05]; the decreases of RR in dexmedetomidine group and midazolam group were significant than that in control group after 1 hour of sedation (bpm: 34.0±3.9, 33.8±4.6 vs. 39.0±3.6, both P < 0.05). There were no differences of MAP, HR, PaO 2 and RR between dexmedetomidine group and midazolam group at different time points. The sleep duration in dexmedetomidine group and midazolam group were significantly longer than that in the control group (hours: 4.9±1.9, 5.8±2.4 vs. 3.0±1.8, both P < 0.05), but there was no difference between dexmedetomidine group and midazolam group (P > 0.05). Adverse events occurred in all three groups. In midazolam group, there were 2 cases of excessive sedation with fall of tongue and abdominal distension, including 1 case of aspiration, 1 case receiving intubation due to refractory hypoxemia and 1 case due to unconsciousness. In dexmedetomidine group, there were 2 cases of bradycardia, 1 case of intubation due to refractory hypoxemia. In control group, 4 cases underwent intubation due to refractory hypoxemia., Conclusions: Non-invasive mechanical ventilation is an important respiratory support technology for patients with severe COVID-19. Appropriate sedation can increase the efficiency of non-invasive mechanical ventilation. Dexmedetomidine is more effective and safer than midazolam in these patients, but attention should be paid to HR and blood pressure monitoring.