Pleural thickening refers to the phenomenon of pleural hyperplasia, fibrosis and calcification, which increases the thickness of the pleura and may be due to a variety of causes such as pleural inflammation, tumor lesions and connective tissue disease. Mild thickening is not serious, but severe thickening will affect the respiratory function of the lungs and needs attention. 1, pleural inflammation: such as tuberculous pleurisy, exudative pleurisy, purulent pleurisy, etc. Inflammation will stimulate pleural proliferation and fibrosis for a long time, and the new pleural tissue will cover the original pleura, causing pleural thickening. 2, tumor lesion: due to the continuous reproduction of tumor cells, it will lead to continuous pleural thickening. In most cases, it is diffuse pleural thickening, which is related to the primary malignant tumor of pleura, such as infiltrative growth of malignant pleural mesothelioma, or extensive metastasis of metastatic cancer. Benign pleural tumors are less common, mostly limited pleural thickening, and larger tumors may affect respiratory function; 3. Connective tissue disease: Some connective tissue diseases, if occurring at the pleura or involving the respiratory system, can cause fibrotic lesions of the pleura, such as systemic lupus erythematosus and giant cell arteritis. Imaging examinations mostly suggest diffuse pleural thickening, and some patients have limited foci of calcification; 4. Other: pneumoconiosis, pneumonia, hemopneumothorax, chest trauma and other lesions can also cause pleural thickening, and the pleura is gradually thickened by continuous fibrosis and calcification in the course of the patient’s lesion, as well as being cured. Most cases of mild pleural thickening are not serious and do not need to be treated; the lungs are able to compensate after the primary disease is cured, making the symptoms disappear. Severe pleural thickening can affect respiratory function and requires surgical pleural debridement, which is more critical.