Hydrothorax is a condition in which there is an increase in the production of fluid or a decrease in the absorption of fluid in the pleural cavity due to various reasons, and the amount of fluid that is stored exceeds the normal amount in the body. According to the mechanism of occurrence can be divided into two categories: exudative pleural effusion and leakage pleural effusion. 1. Exudative pleural effusion (1) Increased permeability of pleural capillaries: such as pneumonia, tuberculosis, pleural mesothelioma, pleural metastasis, connective tissue disease, pulmonary embolism, subphrenic inflammation, etc. can cause increased permeability of capillaries leading to increased production of fluid to appear pleural effusion. (2) Impaired lymphatic return in mural pleura: obstruction of lymphatic vessels in cancer, abnormal drainage of developmental lymphatic vessels, etc. may lead to decreased absorption of fluid in the pleural cavity and pleural effusion. 2.Leaky pleural effusion (1) Increased hydrostatic pressure in pleural capillaries: cardiac insufficiency, constrictive pericarditis, superior vena cava syndrome or odd vein obstruction, etc. lead to increased fluid production and pleural effusion. (2) Decreased colloid osmotic pressure in pleural capillaries: e.g., hypoproteinemia, cirrhosis, nephrotic syndrome, mucous edema, etc., leading to decreased absorption of fluid in the pleural cavity to appear pleural effusion. There is fluid in the pleural cavity, it is recommended to go to the hospital for further diagnosis and treatment.