Pulmonary effusion is a collection of fluid in the pleural cavity. Pleural effusion usually does not cause serious consequences and there is a small amount of fluid normally in the pleural cavity. It acts as a lubricant between the wall pleura and the dirty pleura. The organism is in a dynamic equilibrium through certain mechanisms for the continuous filtration and absorption of fluid in the thoracic cavity. Pleural effusion occurs when there is an increase in the production of pleural fluid or a decrease in its aspiration. When the amount of pleural fluid is small, there are no obvious clinical symptoms. When the amount increases, symptoms of chest tightness and gas compulsion will appear after drainage by tube placement, and the symptoms of draining pleural fluid can be rapidly relieved. According to the examination of pleural fluid and blood, combined with chest imaging, the cause can usually be found to get effective and will not cause serious adverse consequences.