Abstract: This paper derived the locus equations of any point on the wheel foot and the enveloping curve equation of driving surface of the wheel foot, and then analized the-shape and size of the wheel foot hole outline on soil, and the length and location of the enveloping curve within the hole outline, and furthermore presented the calculating method of the contact length between the soil and the wheel foot driving surface when the walking wheel runs on soft ground. All of these is essential to the further study of the kinetic performance of the walking wheel.