Left upper arm lateral pain is considered to be the cause of cervical spondylosis, rotator cuff injury, muscle strain, myofibrillitis and so on. 1. Cervical spondylosis compresses the nerves, which can involve the pain on the outside of the left upper arm, and some of them are accompanied by numbness, and the pain can radiate to the fingers, which can be clearly diagnosed by doing cervical spine MRI films. 2. Rotator cuff injury: rotator cuff injury occurs because of ligament tear caused by shoulder degeneration or trauma, which can cause pain on the lateral side of the left upper arm and obvious limitation of shoulder joint activities, and can be clearly diagnosed through MRI films of shoulder joint. 3. Muscle strain, if often do tired work or a posture process can cause upper arm deltoid muscle strain, pain phenomenon, can be reduced after rest. 4. Myofibrositis: aseptic inflammation of the myofascia, manifested by pain, swelling, and restricted movement. So the lateral left upper arm pain is generally considered the above reasons, the appearance of the lateral left upper arm pain and persistent unrelieved, need to consult a doctor in a timely manner, according to the cause of the treatment.