A numerical simulation of collision between a 1000t ship and a floating anti-collision steel box of bridge is carried out using Ansys/Ls-dyna. In order to figure out the influence of the flow field on the results of the numerical simulation, fluid-structure coupling method and the added mass coefficient method is applied respectively to account the effect of the flow field during collision process and the results from the two methods are compared, such as collision force, collision depth, vertical displacement and internal energy change of the anti-collision steel box. Results show that, while considering fluid-structure interaction, the maximum collision depth is smaller, meanwhile the maximum collision force is larger, and the maximum collision force is significantly larger with the collision energy increasing, compared to the added mass coefficient method. Therefore, considering fluid-structure interaction to account the effect of the flow field is necessary for the anti-collision steel box design of bridge. Furthermore, from results, the larger ship collision energy is, the more important vertical displacement is. By designing the ballast water capacity appropriately, the vertical displacement of steel box during collision process can be reduced and the energy-absorbing capability can be improved.