The most common reasons for pleural fluid in lung cancer are two: first, pleural metastasis of lung cancer. Lung cancer patients are prone to pleural metastasis, and cancer cells transferred to pleura will stimulate pleura to secrete a large amount of exudate to the chest cavity, forming malignant pleural fluid. Hypoproteinemia, when lung cancer reaches the advanced stage, patients have poor nutritional status due to loss of appetite and huge consumption of malignant tumors, and the concentration of albumin in blood is very low, which leads to the decrease of plasma colloid osmotic pressure, and the water in the blood will leak out to the chest cavity and form pleural effusion, which is different from the exudate caused by metastasis and is called leaking effusion. Other uncommon causes include lung infection. Lung cancer patients’ lungs are often obstructed by tumors, leading to severe obstructive pneumonia, and the infection will also stimulate pleura to secrete exudate into pleural cavity to form pleural fluid. In conclusion, the causes of pleural effusion in lung cancer patients are multi-faceted, often caused by the combination of the aforementioned reasons, and rarely caused by a single factor.