Inclusion Criteria:  1.85 years≥Age≥18 years. 2.Moderate-to-severe ARDS, defined by the ARDS Definition Task Force in the Berlin definition (partial pressure of arterial oxygen [PaO2]:FiO2 ratio ≤200 mmHg with a PEEP ≥5 cmH2O).  3.Diagnosis of ARDS less than 72 hours.  Exclusion Criteria:  All patients who meet any of the following criteria will be excluded at enrollment and randomization.  Severe chronic respiratory diseases requiring long-term home oxygen therapy or noninvasive MV. Undrained pneumothorax or subcutaneous emphysema. Severe neuromuscular disease. Hemodynamic instability (>30% increase in vasopressors within 6 hours or norepinephrine >0.5 ug/kg/min) (Am J Respir Crit Care Med. 2020; 201(2):178-187) . Contraindications to hypercapnia, such as intracranial hypertension or acute coronary syndrome. Severe other organs dysfunction with a low expected survival (7 days) or palliative care. Solid organ or hematologic tumors with the expected survival time less than 30 days. Participating in other clinical trials within 30 days. Pregnancy. Refusal to sign the informed consent.