Inclusion Criteria:  We included patients with acute respiratory distress according to 2012 ARDS Berlin new definition (Acute Respiratory Distress Syndrome: The Berlin Definition. JAMA, 2012, 307(23):2526).  Acute or progressive dyspnea within 1 week with identify cause; Chest radiograph/chest CT showed double lung infiltration, which could not be fully explained by pleural effusion, atelectasis, or nodules; Respiratory failure cannot be fully explained by heart failure and fluid overload; Hypoxemia, partial pressure of oxygen in arterial blood (PaO2)/oxygen fraction in air (FIO2) <150 mm Hg under PEEP ≥5 cm H2O, (mild ARDS: 200mmHg<PaO2/FiO2≤300mmHg, moderate ARDS: 100mmHg<PaO2/FiO2≤200mmHg, severe ARDS: PaO2/FiO2≤100mmHg); 18~70 years old; Agree to participate in the trial, and sign the informed consent.  Exclusion Criteria:  Age less than 18 years old; Time of hospital stay <24 h; Pregnancy; Using V-V ECOM; Cardiac index <1.5L·ml.min-1.m-2; Pulmonary resection; Pulmonary embolism ; Refused to participate in the study.