The flexible floating collision-prevention system (FFSCS) is a valuable floating engineering structure that can be used to prevent the uncontrolled ships to collide with the non-navigational bridge of a large sea-crossing bridge. The system is composed of buoys, block chains, mooring chains and gravity anchors. The deformation of the system under the acting of an uncontrolled ship as well as the movement distance of the gravity anchors are important factors that should be considered by the system designers. Based on the analysis of the relationship between the forces and the deformation of each part of the system, the approximately static equations are solved by a new numerical iterative calculation method. The position changes of the buoys, the movements of the anchors and the history of the inner forces of the block chains when a ship collides with the FFSCS are obtained by iterative calculation. The good agreement between the numerical value and the results of the model test indicate that the small balance method is a validation on the motion response simulation of the FFSCS under the acting of the uncontrolled ship. The results validate that FFSCS can stop the uncontrolled ship before it arrives at the place of the bridge.