Half of the arm is numb, commonly due to cervical spondylosis, elbow canal syndrome, ulnar canal syndrome and other causes, as follows: First, cervical spondylosis: neurogenic cervical spondylosis or mixed cervical spondylosis can cause symptoms of numbness in half of the arm of one upper limb, mainly due to nerve compression. Patients will be accompanied by neck pain and limited cervical movement, and cervical MRI will reveal cervical lesions. Second, elbow canal syndrome and ulnar canal syndrome: left elbow canal syndrome and ulnar canal syndrome can cause numbness in half of the arm, accompanied by muscle atrophy, and electromyography of the upper limb will reveal the location and severity of the lesion. Patients with half arm numbness should be diagnosed and treated early, and the nerve conduction function can be restored only through conservative treatment or surgical treatment to completely release the nerve compression and avoid serious arm muscle atrophy and upper limb dysfunction. Patients should usually pay attention to protecting the cervical spine, elbow joint and wrist joint to avoid nerve compression.