The causes of pain in the rib area under the left armpit may be left-sided rib fracture, costochondritis, malignant tumor of the left lung and so on. 1. Left side rib fracture: when the left side rib is traumatized, it can cause left side rib fracture. It leads to obvious pain and activity limitation in the rib area under the left armpit. At this time, it is recommended to go to the orthopedic department of the hospital as soon as possible for X-ray or CT examination, so as to facilitate the development of the best treatment plan. 2. Costochondritis: It is a chronic non-specific inflammation that occurs in the rib cartilage area. For example, lifting heavy objects and twisting them sharply, squeezing the chest, and chronic strain injury can cause acute and chronic damage to the cartilage of the thoracic rib joints and lead to costochondritis, which will cause pain in the rib area under the left armpit. 3. Malignant tumor of left lung: when cancer cells invade into chest wall or ribs, it can cause pain in the rib cage area under left armpit. At this time, it is necessary to improve lung CT as soon as possible to clarify the exact location of the tumor, so as to facilitate the formulation of the best surgical plan. 4. Others: such as lung infection, pleurisy, intercostal neuritis, etc., can lead to pain in the left axilla rib cage area. It is recommended to pay attention to rest, avoid late night and labor, and avoid mental stress. When the symptoms are serious and cannot be relieved, it is recommended to consult a doctor in time to avoid delaying the condition.