Left shoulder pain can be caused by lung cancer or other diseases. When there is lung cancer, shoulder pain can appear on one side, especially in the tip of lung or upper lobe of lung, shoulder pain can appear reflexively, and CT of lung should be performed for early diagnosis and treatment. Besides lung cancer, there can be other reasons leading to left shoulder pain, as follows: firstly, cervical spondylosis, which can compress the nerve root and lead to shoulder pain, and then X-ray, CT and nuclear magnetic examination of cervical spondylosis should be carried out for diagnosis; secondly, shoulder diseases, when rotator cuff tear or periarthritis occurs in the shoulder joint, shoulder pain can occur, and then X-ray, CT and nuclear magnetic examination of shoulder joint should be carried out for early diagnosis and treatment. Secondly, shoulder disease, when rotator cuff tear or frozen shoulder, shoulder pain can occur, then X-ray, CT, nuclear magnetic examination of shoulder joint should be carried out for diagnosis; thirdly, it can also be caused by the nerve compression of shoulder, for example, the suprascapular nerve is compressed and shoulder pain occurs, then relevant examination and electromyography examination should be carried out for diagnosis.