Free-standing rack system, where each rack is not fixed to the floor nor the wall, is proposed and is in use in European countries and the US as a storage method of the nuclear power station spent fuel. Although this system can reduce the influence of the excitation force of earthquake by making use of frictional force between the bottom surface of the rack and the floor surface together with the fluid force excited by the motion of each rack, design guidelines are not yet established. In this research, to evaluate the fluid force more precisely, the gap between the racks is treated as a two-dimensional gap flow, the pressure loss coefficient at the flow path junction and the top of the flow path were estimated based on the steady CFD calculation and incorporated with the motion model. Our main concern in this paper is the rocking motion. As a result, it was concluded that rocking motion can be suppressed by increasing the pressure loss coefficient at the top of the fuel rack.