Based on the consideration of the pile-soil nonlinear interaction, nonlinear equivalent spring is introduced in this paper to simulate the action of soil on piles.

Meanwhile, the natural frequency was calculated using this method in an example, and the natural frequency affected by the change of equivalent spring stiffness was also analyzed.

Dynamic differential equation is established with Kane method. The formulas of input torque at each joint and force acting on the ground and the approach to equivalent spring stiffness were presents.