Sudden left shoulder pain may have the following reasons: first, cervical spondylosis. When cervical spondylosis compresses the nerve root, sudden pain in the left shoulder will occur, and then relevant cervical spine X-ray and nuclear magnetic examination should be carried out for diagnosis and treatment. Second, shoulder diseases. When rotator cuff tear occurs in the shoulder joint, sudden pain in the shoulder joint will occur, and then relevant nuclear magnetic examination of the shoulder joint should be carried out for diagnosis. Third, myofasciitis. Myofasciitis can also lead to shoulder joint pain, and it is necessary to carry out relevant physical examination to exclude other diseases for diagnosis. Fourth, heart disease. When the heart has heart attack or angina pectoris, the pain in the left shoulder will appear reflexively, and then blood test, heart ultrasound and electrocardiogram should be carried out for diagnosis and treatment.