Acute respiratory distress syndrome (ARDS) and the serious complications are the most frequent causes of death of SARS-CoV-2 infection. We bring out a hypothesis that early low-flow oxygen inhalation would maintain the hypoxic pulmonary vasoconstriction (an essential protection mechanism of the lung that optimize gas exchange) and accelerate the re-absorption of pulmonary edema fluid. The optimal time for oxygen therapy was analyzed and four comments are proposed: (1) Finger SpO2 should be measured at home simultaneously with the first-time nucleic acid test. (2) If the patient's SpO2 was lower than the reference value by 2% or more, it is suggested to be hospitalized immediately for standard low-flow oxygen inhalation. (3) If it was not possible to be admitted to hospital immediately, the patient is recommended to take oxygen in the home. (4) The Patients with low SpO2 are advised to use prone position as much as possible.