Localized thickening of the left pleura is caused by an increase in pleural exudate due to pleural lesions or other etiologies, which leads to granulation tissue proliferation and ultimately pleural thickening. Causes of partial thickening of the left pleura include pleurisy, tumor lesions, and other pathologies. 1. Pleurisy: such as purulent pleurisy, tuberculous pleurisy, exudative pleurisy, etc., which can stimulate the pleura for a long time and cause local thickening of pleura after granulation tissue proliferation and fibrosis. 2. Tumor lesions: such as pleural metastasis, malignant pleural mesothelioma, etc., which can cause the increase of pleural exudate, causing local pleural thickening. 3. Other lesions: such as chest trauma, pneumonia, etc., can stimulate the pleura to appear localized pleural thickening. Localized pleural thickening symptoms, need to go to the hospital in time for examination, identify the cause of the disease, targeted treatment.