Background　Permissive hypoxemia is a lung-protective strategy that aims to make critically ill patients with refractory hypoxemia maintain normal levels of oxygen delivery through optimizing cardiac output（CO） to partially compensate respiratory function in a relatively low oxygen saturation， and thereafetr minimize the detrimental pulmonary and systemic effects of ventilatory support.　Objective　The objective of this review is to offer reference for the understanding and discussion of the rationality and progression of permissive hypoxemia.　Content　Elaborating the concept， origin， rationality and progression of permissive hypoxemia， and problems that may be needed to concern in clinic.　Trend　Theoretically， permissive hypoxemia may improve outcomes in critically ill patients. Clinical studies are clearly needed to evaluate the safety and efficacy of permissive hypoxemia.