The pain in the armpit against the back is considered to be caused by muscle strain, fasciitis, lymphadenitis and other diseases. 1. Muscle strain: the disease is mainly due to the patient’s recent frequent muscle exercise or physical labor, resulting in frequent muscle contraction, stretching, beyond its capacity muscle tear or even rupture caused by the patient often lead to local pain, soreness, redness and swelling and other symptoms. If the patient recently dragged heavy objects and other actions, resulting in underarms against the back of the local muscle strain, there will be pain symptoms. 2. Fasciitis: this disease is a kind of aseptic inflammation that occurs in the muscle fascia, the specific cause of which is still unclear, clinically considered to be related to local muscle strain, trauma and other factors. The disease can occur in all parts of the body myofascia, preferably in the shoulders and neck, back and other locations, often leading to pain, muscle weakness, numbness and other symptoms, some patients may be manifested as armpit by the back of the place of pain. 3. Lymphadenitis: this disease is more common in the clinic, mainly due to bacterial infection, often resulting in patients with lymph node enlargement, pain and other symptoms. Normal people have more lymph nodes in their armpits, and if the patient has a recent case of inflammation of the lymph nodes in the armpits, the symptom of pain in the armpits against the back will occur. There are many clinical causes of axillary back pain, so it is recommended that patients seek medical attention to determine the specific cause of the condition.