Weakness of legs and sudden fall may be spinal cord cervical spondylosis, periodic paralysis, transient ischemic attack. 1. Spinal cord cervical spondylosis: cervical spondylotic hyperplasia, spinal stenosis or cervical spondylolisthesis compresses the spinal cord, causing spinal cord edema and degeneration, which results in lower limb weakness, the feeling of stepping on cotton, and easy to fall. With the development of the disease, the symptoms will gradually aggravate, and even can not walk, through the cervical spine MRI can be clearly diagnosed, through surgical treatment to relieve the compression of the spinal cord symptoms can be relieved. 2. Periodic paralysis: the common cause is the excessive loss of potassium, such as excessive sweating or diarrhea caused by the patient, mainly manifested by the weakness of the lower limbs and easy to fall, through the blood potassium test can be clearly diagnosed, supplement potassium chloride symptoms can be improved. 3. Transient ischemic attack: dizziness, weakness, nausea, vomiting, tinnitus and sudden fall. Therefore, the above three conditions should be considered for the sudden fall of weak legs, and it is better to go to the hospital for clarification and early treatment to avoid further aggravation of the symptoms.