The causes of sudden swelling of the left foot are as follows: 1, it may be due to soft tissue injury caused by sprains, caused by simple symptoms of swollen feet. 2, it may be due to lower extremity venous thrombosis caused by swollen feet, you need to go to the regular hospital to do vascular ultrasound examination in a timely manner to make a clear diagnosis. 3, it may be due to hypo-proteinemia, low-protein tissues with reduced osmotic pressure, which requires water to flow out of the tissues, thus causing tissue edema, even In severe cases, swelling can occur. 4, may be caused by systemic reasons, such as cardiac insufficiency, severe hypoproteinemia, cardiac insufficiency affects the return of blood, the pressure inside the venous blood vessels increases, the water outflow, can cause the lower extremities of the swelling symptoms.