The pleural cavity is a potential cavity formed by the dirty pleura and the wall pleura, often consisting of only 5-10 ml of a small amount of synovial fluid. When pleural effusion appears, it usually indicates that the pleural cavity is diseased, and the color and cause of pleural effusion are as follows: 1, when the effusion is pale yellow and clear, it is often considered to be leakage fluid, which may be caused by cardiac insufficiency or hypoproteinemia; 2, when the pleural effusion is grass green, and the patient is accompanied by symptoms of tuberculosis poisoning such as low fever, weakness and night sweats, it is often considered to be tuberculous pleural effusion 3. When pleural effusion is old and bloody or bloody effusion, malignant pleural effusion is often considered, such as lung cancer, or pleural metastasis caused by cancer from other parts. Therefore, when pleural effusion appears, the cause should be clarified and actively treated.