Abstract: The mechanism of the deformation for a loaded pile is studied in this paper. A-full-depth/part-depth shear spring model is proposed to simulate the constraint of the soil.Based on this model, the equation of the load-settlement curve is derived. The result of computation agrees with the result of static loading test closely.Three dimensionless parameters are also proposed, which may be used to describe the characteristic of a pile-soil system. Meanwhile, the method of calculating the equi-valent spring coefficient is given.