In this paper, we introduce the principle of high speed train eddy current braking, and establish a complete mathematical model for solving eddy current force definite solution problem. The eddy current braking process is a dynamic process. The air gap distance between the magnetic pole and guide rail induction plate is constantly changing, and the change of air gap and the eddy current brake force are mutually coupled. Obviously, the dynamic coupling process of eddy current braking force with air gap cannot be characterized by static analytical expressions. We propose a method to calculate the dynamic simulation of eddy current braking force considering the change of air gap. First, the finite element method is used to establish the 2-dimensional (2D) model, and then the ANSYS software is applied to simulate the calculation. Finally, the results of eddy current brake force (i.e., horizontal braking force) and the normal force change with speed are obtained. We did the analysis on the simulation results, which were further compared with the experimental data. The reasons of the error were analyzed too. This paper provides a reference for the design of high speed train eddy current device.