Lung cancer pleural effusion of red color is also called lung cancer hemorrhagic pleural fluid, which is mostly due to invasion of pleura by lung cancer lesions. When lung cancer invades into pleural cavity, cancer cells will stimulate pleura to over-secrete a large amount of exudate to form pleural effusion, and if blood vessels in pleura rupture and bleed due to stimulation, the exudate will contain a large amount of blood and water, which will result in red color of pleural effusion of lung cancer. Besides, the invasion of cancer cells into pleural tissues will lead to the increase of permeability of pleural cavity, so that too many red blood cells can enter into pleural cavity through pleura, thus leading to the red color of pleural effusion of lung cancer patients. When the color of pleural effusion of lung cancer is red, it suggests that the patient’s condition may enter the advanced stage and the lung cancer has metastasis in the chest. It is suggested that patients should go to the hospital in time and follow the doctor’s instruction after clarifying the specific cause of the disease, so as to avoid delaying the disease and affecting their life.