The stage of lung cancer patients with pleural effusion needs to be assessed based on the pathological examination of pleural effusion as well as pleural examination. If the pleural effusion can detect cancer cells after thoracentesis aspiration and examination of exfoliative cytology, it means that lung cancer with pleural metastasis is in advanced stage of lung cancer. If cancer cells cannot be detected by multiple pleural fluid examinations, thoracoscopic examination can indicate multiple cancerous nodules on the pleura, which is also a manifestation of advanced lung cancer. If the pleural effusion examination and pleural examination cannot detect cancer cells, there is another case of suspected malignant pleural effusion, which is mainly due to the compression of lymphatic circulation by lung tumor, and it is earlier than lung cancer pleural metastasis and does not belong to the middle and late stage.