A computational study was carried out in order to investigate the ground effect of a circular cylinder in the presence of a moving wall at a Reynolds number of 2.0${ imes}$104. The viscous-incompressible Navier-Stokes equations and Spalart-Allmaras turbulent model of the commercial CFD code were adopted for this numerical analysis. The moving wall was set parallel with the freestream, and the speed of motion was equal to the freestream velocity. The gap ratio is defined as the distance ratio between the circular cylinder diameter and the height from the moving wall. The numerical results show that there are the differences among the each of the stages in evidence of the vorticity contours and the polar diagrams of $C_l$ vs. $C_d$. The 4 stages of the gap ratio are defined according to the flow features, whose stages are divided into small, intermediate, large and convergence gap ratios, respectively.