Alveolar lavage fluid is inserted directly into the airway with a fiberoptic bronchoscope to reach the site of inflammation or lesion, sterile saline is injected through the fiberoptic bronchoscope and repeatedly aspirated, and the final aspirated fluid is called alveolar lavage fluid. In clinical practice, alveolar lavage fluid is mainly used for pathogenic microbiological examination. The fluid from alveolar lavage is aspirated into a sterile bottle and a series of laboratory tests are done in the laboratory, and inflammatory substances or tumor cells can be found in the lavage fluid of certain immune diseases or even tumor diseases.