Myofasciitis can be associated with pain in the medial side of the left scapula and numbness and pain in the left upper extremity, which is more pronounced when cold or under pressure at night. Second, heart disease, when the heart has a heart attack or angina pectoris, reflexive pain in the back of the left shoulder, then electrocardiogram and ultrasound imaging of the heart or blood tests should be performed to further diagnose and treat. Third, rib and lung diseases, if there is a tumor in the ribs or a tumor in the upper lobe of the lung, the pain in the back of the left shoulder can also occur. Fourth, cervical spondylosis, when cervical spondylosis compresses the nerve or cervical spondylosis has a muscular type of cervical spondylosis, the pain in the back of the left shoulder can occur, then the relevant cervical spine X-ray and nuclear magnetic examination should be performed for further diagnosis.