Pulmonary effusion is generally defined as a collection of fluid in the chest cavity, also known as pleural fluid. Whether a pulmonary effusion is serious or not needs to be judged in relation to its etiology, the amount of effusion and the cause of the effusion. Generally a small amount of fluid in the lungs is not a major problem in most cases. To moderate and large amounts of fluid in the lungs, especially when it affects normal breathing and even circulation, may be dangerous and usually serious. Most clinical cases of pulmonary effusion are caused by common bacterial infections and are treated with antibiotics for about 2 weeks, and the effusion can be significantly absorbed and is mostly not serious. In the case of tuberculous pulmonary effusion, the treatment period may be longer, even up to six months or more, and relatively more serious. Pulmonary effusion caused by malignant tumors requires different treatment methods according to the primary disease. Squamous lung cancer can be actively controlled through surgery, while small cell carcinoma or adenocarcinoma of the lung requires corresponding surgery, chemotherapy and radiotherapy according to its staging, which can prolong patients’ life and improve their quality of life. The common symptoms of pulmonary effusion are cough, fever, chest distension and dyspnea, etc. Early detection, diagnosis and treatment are beneficial to the control of the disease; the later the detection, the more difficult the treatment.