Animal experimentals have shown that the more physiology-driven airway pressure release ventilation (APRV) methodologies in ARDS may significantly improve alveolar recruitment and gas exchange, increased homogeneity, and attenuate lung injury, without circulatory depression, as compared with conventional low tial volume lung protective ventilation. our previous single centre,random control study showed that clinical benefit for early use of APRV in ARDS. Nonetheless, clinical data on ARDS are still limited, most of them derived from small clinical trials in which variable outdated APRV settings were used, consequently, the findings of these studies were controversial.

Additionally, the previous single-centre,random control study showed that clinical benefit for APRV.Therefore,the investigators are ready to design a multiple centres,random control study to further verify the effect of APRV plus protocol in ARDS.

All the patients included will be randomly assigned to receiving APRV plus protocol or low tidal volume ventilation.

Inclusion Criteria:

• Moderate and severe acute respiratory distress syndrome,according to the Berlin definition of ARDS
• receiving tracheal intubation and mechanical ventilation was no longer than 48 hours

Exclusion Criteria:

• Pregnancy
• The expected duration of mechanical ventilation was less than 48 hours
• Intracranial hypertension (suspected or confirmed)
• Neuromuscular disorders that are known to prolong the need for mechanical ventilation
• Known or suspected chronic obstructive pulmonary disease(COPD)
• Terminal stage of disease
• Pneumothorax (drained or not)at enrollment
• Treatment with extracorporeal support (ECMO) at enrollment
• There was a lack of commitment to life support.