The common causes of left lower lung field shadow include pneumonia, tuberculosis and lung tumor. 1. Pneumonia: patients with chest CT or chest radiographs of the lung field strip shadow, need to further routine blood, C-reactive protein and other tests, there may be cough, sputum, fever and other manifestations, need to choose sensitive antibiotics for anti-infective treatment, such as cephalosporin penicillin class antibiotics. 2. Tuberculosis: patients with pulmonary tuberculosis have lung field film on chest X-ray, they need to find antacid bacteria in sputum to give a clear picture, and give anti-tuberculosis drugs such as rifampicin and isoniazid, and they need to pay attention to the blood test to check the liver function because anti-tuberculosis drugs have a big effect on the liver function. 3. Lung tumor: Lung cancer patients should have chest CT with streak-like shadow in lung field, sputum search for cancer cells, check tumor markers to confirm the diagnosis, and undergo surgical resection for confirmed lung cancer. If the patients have any discomfort, they should go to the hospital in time and avoid carelessness so as not to delay the treatment.