<p>The violent free-surface flow is a common phenomenon in ocean engineering. Damping the violent free-surface flow is crucial for the safety and stability of marine structures. Therefore, this study investigated the damping effect of floating particles on the free-surface flow in a container. A coupled numerical model, including the discrete element method (DEM) and smoothed particle hydrodynamics (SPH), was adopted to calculate the interaction between floating particles and the free surface. Various validation tests were conducted to verify the reliability and accuracy of the coupled DEM-SPH method. A parametric study considering various parameters of floating particles was conducted. The damping rate of the free surface, velocity, and kinetic energy of the flow field was adopted to evaluate the damping effect of floating particles on the free surface. The results show that the floating particles can effectively dampen the free-surface flow and weaken the nonlinear effect of the free surface. The density and layer of floating particles and the friction coefficient between floating particles and boundary walls play an important role in damping the free-surface flow. Furthermore, floating particles with smoother surfaces exhibit superior damping performance. An optimal configuration of floating particles was proposed and demonstrated to exhibit excellent damping performance. The results of this study have practical value for the design of liquefied cargo containers.</p>