The principle of oxygen therapy for type II respiratory failure is continuous low-flow oxygen, which is mostly caused by chronic lung diseases, such as chronic obstructive pulmonary disease, asthma, etc. Long-term carbon dioxide retention produces inhibition of the respiratory center, and the maintenance of respiration relies on the stimulation of the peripheral chemoreceptors by the lack of oxygen, and once the high-flow oxygen is given, the peripheral chemoreceptors’ response to the lack of oxygen decreases in a short period of time, which causes respiratory inhibition and carbon dioxide retention. Nasal catheter oxygen is generally used, oxygen flow rate 1-2L/min, oxygen time＞15h/d. The purpose is to achieve PaO2≥60mmHg and/or increase SaO2 to more than 90% when the patient is at sea level and in a resting state.