Weakness of legs in the elderly should first clarify the cause of the disease, and then treat the cause of the disease. Leg weakness may be caused by potassium deficiency, calcium deficiency, lumbar spondylosis, etc. If it is caused by potassium deficiency, it can be treated by potassium supplementation under the guidance of doctor. The normal blood potassium of human body is 3.5~5.5 mmol/L. Hypokalemia may cause symptoms, including weakness, muscle weakness, nausea and vomiting, arrhythmia, and increased nocturia. When the elderly have symptoms of leg weakness, they should go to the hospital for laboratory tests. If it is confirmed that the leg weakness is caused by hypokalemia, they should be treated with potassium supplementation in a timely manner. For mild hypokalemia, foods high in potassium can be consumed, such as bananas, oranges, potatoes, sweet potatoes and other foods rich in potassium, and potassium chloride extended-release tablets can be taken orally. In case of severe hypokalemia, intravenous potassium supplementation, such as potassium chloride solution added to glucose solution, is needed. Potassium supplementation needs to be treated with medication under doctor’s guidance, and blood potassium indexes need to be tested at the same time, so as to avoid oversupplementation of potassium leading to hyperkalemia.