 Ninety-three patients with ARDS will be enrolled . These patients are treated routinly.They need to recept a time of ZEEP PEEP is zero cmH2Ofor ten respiratory cycles before RMrecruitment maneuvers.Implemente a kind of RM at random immediately after ZEEP,Then make the ventilator same as before ZEEP.The EIT images ,blood gases and hemodynamic variables will be examined as beseline between ZEEP and RM.Completion of this kind of RM lies that the PF of blood gases 3 minutes after RM achieves 1 PO2FiO2400mmHg or 2PO2FiO2PCO2 400mmHgFiO21,3the difference between this index before lung recriument and after lung recriument PO2FiO25.In addition,3 minutes after RM,compare the indexs above between three methods. 