Left upper limb pain can be caused by the following reasons: First, cervical spondylosis. When cervical spondylosis compresses the nerve roots, there can be radioactive pain on one side of the upper limb, which should be investigated early, and cervical spine X-ray, CT, MRI should be checked early for early diagnosis and treatment. Second, periarthritis or rotator cuff tear, when periarthritis or rotator cuff tear, there can also be radiating pain in the upper arm or forearm, then early examination of the MRI of the shoulder joint is needed for diagnosis and treatment. Third, myofasciitis, which can radiate to the ipsilateral upper extremity, with symptoms of soreness and weakness, and can be accompanied by pain in the upper extremity, which is very obvious when exerting or sleeping under pressure, should be checked and examined early to exclude other diseases for diagnosis. Fourth, head diseases, when there is a tumor in the head compressing the nerve, it can also have the symptoms of upper limb pain, then we should check the MRI of the head early to diagnose and treat.