The pain below the left rib cage when taking a deep breath is caused by the large expansion of the thorax when taking a deep breath, and the nociceptive nerve endings in the periosteum of the rib cage and the wall pleura are once again stimulated by inflammation or pulled, and so on. It is commonly seen in rib fracture, costochondritis, intercostal neuritis, pleural and pleural cavity diseases, lung diseases and so on. 1. Left rib fracture: deep breathing can cause relative displacement of the fracture end and stimulate the nociceptive nerve endings in the periosteum, causing pain below the left rib. 2. Left costochondritis: chronic non-specific inflammation, coughing and deep breathing can induce and aggravate pain in the costochondral area. 3. Left intercostal neuritis: it can cause pain above and below the ribs at the innervation of intercostal nerves, which is aggravated when the patient breathes deeply or sneezes. 4. Left pleural and pleural cavity diseases: left pleurisy, left pleural mesothelioma, left pneumothorax, left pleural effusion, etc., the chest pain below the left rib cage is felt when breathing, and it is obvious when deep breathing or coughing. 5. Lung diseases: when there is inflammation, tuberculosis, tumor in the left lung, or left pulmonary embolism, left chest pain can also be felt when breathing hard. Deep breathing pain below the left rib cage should go to the hospital for further examination to clarify the cause of the disease and then further treatment.