Sudden weakness in the legs and inability to walk in small children may be caused by excessive activity, hypoglycemia or hypokalemia, mild viral myositis, and Guillain-Barre syndrome, which require treatment for the cause. 1. Excessive activity: children play too much activity, resulting in muscle pain and sudden weakness, more rest can relieve. 2. Hypoglycemia or hypokalemia: children with sudden transient hypoglycemia will have weak legs, darkness in front of their eyes, or even fainting, which can be improved by eating and resting; vomiting or diarrhea can lead to electrolyte disorders, resulting in hypokalemia and weakness, which can be recovered by replenishment of electrolytes. 3. Mild viral myositis: after children’s respiratory tract is infected by viruses, the viruses invade the leg muscles and there will be weakness in the legs, unable to walk, usually rest for 5 to 10 days can be relieved. 4. Guillain-Barre Syndrome: symmetrical weakness of the limbs starting from the toes or fingers, can be selected under the guidance of the doctor for intravenous injection of immunoglobulin, vitamin B, vitamin C and coenzyme Q10 supplementation. The baby appeared suddenly weak legs can not walk, should be timely consultation, under the guidance of the doctor to clarify the cause of the disease, standardized treatment.