Background: Remimazolam and dexmedetomidine (Dex) are commonly used sedative medication. However, the effects and safety of remimazolam alone or in combination with Dex has not been investigated.Aim: This study aimed to investigate efficacy and safety of remimazolam alone or plus Dex for sedation during bronchoscopy.Methods: This trial enrolled 90 eligible patients undergoing bronchoscopy under intravenous anesthesia. All patients were randomly separated into three groups: group C (propofol), group R (remimazolam) and group RD (remimazolam plus Dex). Primary outcome was incidence of perioperative hypoxemia. Secondary outcomes included induction and maintenance doses of remimazolam, hemodynamic variables, modified Observer's Assessment of Alertness/Sedation (MOAA/S) score, coughing score, limb movement score, incidences of adverse events, patient satisfaction score, bronchoscopist satisfaction score, incidence of postoperative cognitive dysfunction (POCD), time to loss of consciousness (LoC) and time to awake.Results: Incidences of hypoxemia and hypotension, and bronchoscopist satisfaction score were significantly decreased, time to LoC and time to awake were markedly longer in the R group and the RD group compared to the C group (p<0.05). The R group had significantly decreased induction and maintenance doses of remimazolam, and a shorter time to LoC in comparison with the RD group (p<0.05). Coughing score, limb movement score, MOAA/S score, and postoperative patient satisfaction score were comparable in the three groups. POCD was not induced in each group.Conclusion: Co-administration of Dex reduced induction and maintenance doses of remimazolam, accelerated its onset of action and did not induce POCD but with longer time to awake.Trial registration: This trial protocol had been registered on Chinese Clinical Trial Registry (ChiCTR2000041435). This study was approved by the Ethics Committee of Tai'an Central Hospital [(2020) Lunshen No. (14)]. Written informed consent was obtained from all participants.