Fluid in the lungs with fluid generally refers to the presence of fluid in the pleural cavity, which can be categorized as exudate or leakage depending on the etiology. Exudate can be caused by diseases that result in increased pleural permeability, including tuberculous pleurisy, systemic lupus erythematosus, pleural malignancy, pulmonary infarction, and liver abscess. Leakage of fluid can be caused by congestive heart failure, constrictive pericarditis, obstruction of the superior vena cava or singular vein, nephrotic syndrome, and cirrhosis, and is associated with increased capillary hydrostatic pressure or decreased colloid osmotic pressure. Symptoms are not obvious when the amount of pleural cavity effusion is small, but a large amount can cause patients to have dyspnea, palpitation and even respiratory failure. Patients with pleural effusion should seek timely medical treatment, identify the cause of the disease, and actively treat it.