Functional residual capacity (FRC) is the amount of residual air in the lungs at the end of expiration calm, which has important physiological functions. In critically ill patients requiring mechanical ventilation, the lung volume at the end of expiration is called the end-expiratory lung volume. There are many clinical methods for detecting and monitoring FRC, including CT measurement, helium dilution, nitrogen flushing, electrical impedance tomography and other methods. Among them, the nitrogen flushing in/out technology in the nitrogen flushing method is easy to being. carried out in critically ill patients. FRC can be used to assess the condition of lung diseases in mechanically ventilated patients and guide the designation of personalized protective mechanical ventilation strategies to reduce ventilator-induced lung injury. More and more attention is paid to the clinical application of FRC in mechanically ventilated patients, which is of great significance in terms of disease assessment, lung protection ventilation settings for ARDS patients, and lung condition monitoring during anesthesia. This article reviews the pathophysiological functions, detection methods and application of FRC in critically ill patients.