Positive end-expiratory pressure is a concept of mechanical ventilation. Modern mechanical ventilation is all about positive pressure ventilation, which means that the patient is given positive pressure during inspiration to deliver compressed air and oxygen into the patient’s lungs, and sometimes a relatively small positive pressure is maintained during the patient’s expiration, which is called positive end-expiratory pressure. For mechanically ventilated patients, positive end-expiratory pressure has important implications: 1) to maintain the opening of small airways or alveoli, to avoid collapse of small airways or alveoli at the end of expiration, to improve the contributing factors, and to reduce the patient’s respiratory effort; 2) to increase the functional residual air volume and improve lung compliance; 3) to promote the absorption of edema fluid in the lungs. The reasonable application of positive end-expiratory pressure during mechanical ventilation can improve the efficiency of mechanical ventilation to benefit the patient, but the unreasonable application of positive end-expiratory pressure can, on the contrary, aggravate lung overinflation causing pneumatic injury.