There are many reasons for the pain in the armpit against the back, which may be caused by lymphatic system diseases, trauma, soft tissue infection, intercostal neuralgia, pleurisy, etc. It is necessary to improve the examinations and clarify the cause of the disease by combining with the related symptoms. 1. Lymphatic system diseases: there are more lymph nodes in the affected area, when there are infections and other related diseases in the lymphatic system, it can lead to swelling and pain in the lymph nodes in the armpit, such as axillary lymph node inflammation, lymph node metastasis of breast cancer, lymphatic cancer and so on. 2. Traumatic injury: when the armpit is subjected to external force at the back, it can lead to local soft tissue contusion, muscle strain, etc., and the affected area may have pain, which can be aggravated during activities. 3. Soft tissue infections: such as skin infections, boils, etc., which are mostly caused by bacterial infections, and may be characterized by redness, swelling, heat and pain, and may even form abscesses. 4. Intercostal neuralgia: it can be caused by trauma, infection and other factors invading the intercostal nerves, and the pain is distributed along the intercostal space. 5. Pleurisy: when the pleura of the affected area is invaded by bacteria, tumor, etc., it can cause inflammatory reaction of the pleura, which will lead to pain in the area of axilla against the back. There may be other reasons for the pain in the area of armpit against the back, such as angina pectoris, parapneumonia and scapula fracture and scapula tumor. It is recommended that the patient go to the hospital and complete all auxiliary examinations in order to clarify the cause of the disease.