Pleural effusion is usually caused by increased vascular hydrostatic pressure, impaired lymphatic return and increased vascular permeability. 1. Increased vascular hydrostatic pressure: mainly refers to the imbalance of pressure inside and outside the blood vessels, which is mostly seen in congestive heart failure and constrictive pericarditis, etc., so that the fluid inside the blood vessels leaks into the pleural cavity, thus causing pleural effusion. 2. Obstruction of lymphatic reflux: malignant tumors can block lymphatic vessels, which will lead to obstruction of fluid reflux in the pleural cavity, thus causing pleural effusion. 3. Increased vascular permeability: this condition is mostly seen in tuberculosis, pneumonia, malignant tumors and other diseases, which can make the fluid in blood vessels enter into the pleural cavity, thus causing pleural effusion. It is recommended that patients go to the hospital in time for consultation and standardized treatment after clarifying the cause of the disease.