Inclusion Criteria:  1.≤ 3 days (72h) 2.Age ≥18 years, ≤65 years 3.200<PaO2/FiO2 ≤ 300 4.Chest X-ray/chest CT : infiltrates of both lungs 5.Need assisted ventilation (ventilation or high flow oxygen therapy) 6.No left heart failure, pulmonary edema 7.Agree to participate and signe an informed consent  Exclusion Criteria:  1.Life expectancy <3 months due to non-respiratory failure 2.Patients receiving extracorporeal membrane oxygenation support therapy(ECMO),high frequency oscillatory ventilation 3.History of HIV, malignant tumors, or impaired immune function 4.Patients accepted major surgery in the past 14 days (such as tumor removal, thoracotomy, heart surgery, abdominal surgery, intracranial surgery, or surgery for more than 3 hours, etc.) 5.Pregnancy 6.Have a serious concomitant disease 7.Unable to follow-up 8.History of severe allergic reactions or allergy to saline and serum 9.Already participated in another clinical study within 12 weeks 10.Pulmonary edema caused by other underlying diseases