Objective To sum up our experience of hyperbaric oxygen therapy (HBOT) in an elderly critical coronavirus disease 2019 (COVID-19) patient with endotracheal intubation, providing references for the application of HBOT in COVID-19 treatment. Methods and results The patient was 87 years old male and presented coma symptoms on Feb. 3, 2020. Chest computed tomography (CT) showed multiple small flake fuzzy shadows in both lungs. The nucleic acid test of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in bronchoalveolar lavage fluid was positive on Feb.5 and the diagnosis of COVID-19 was confirmed. After symptomatic and supportive treatment, the patient's condition became stable gradually, and the tracheal intubation was removed on Feb. 22. However, the patient was intubated again on Feb. 24 because of loss of coughing and sputum expelling abilities, and the patient's condition was judged to be critical. On Feb. 29, the patient received HBOT for the first time, and medical staff entered the hyperbaric oxygen cabin through the special channel. After HBOT for four times, arterial partial pressure of oxygen (PaO₂) and arterial partial pressure of carbon dioxide (PaCO₂) of the patient tended to be stable, carbon dioxide retention was alleviated, liver and kidney function improved, and coagulation function recovered. On Mar. 6, SARS-CoV-2 immunoglobulin (Ig) test showed that SARS-CoV-2 IgM was negative and SARS-CoV-2 IgG was positive. The patient was then transferred to general wards. Conclusion HBOT can alleviate CO₂ retention in critical COVID-19 patients, and has a positive effect on reducing hypoxia and protecting important organs. The HBOT infection control procedure is feasible, and the safety of medical staff can be guaranteed by reasonable design.