A regime of highly concentrated sediment transport, or sheetflow, which is observed around coastal surf-zone, has significant impact on a coastal process. However, it is difficult to directly measure an inner-structure of the sheetflow from experimental measurement, because the sheetflow is a phenomenon with highly concentrated fast granular flows. As well as, because the sheetflow is a phenomenon in strong non-linear multi-phase flow, an implementation of numerical simulation is still a challenging problem. In the present research, as one way to the understanding of surf-zone sheetflow mechanism, the numerical simulation for the sheetflow sediment transport is performed in the uniform flow condition, by reference to the previous experimental results. In the present numerical model, to simulate sheetflow under open channel flow with free surface with a high degree of accuracy, the enhanced Moving Particle Semi-implicit (MPS) method, which is one of the Lagrangian models, is used to the liquid phase. On the other hand, the Distinct Element Method (DEM) is applied to the solid-particle phase to track individual particle motion. By coupling these methods, the three-dimensional Lagrange-Lagrange coupling model is prepared for the purpose of computational investigation of an inner structure of the sheetflow layer. From the present numerical results, the significant collision frequency between particles is found in the sheetflow layer including a high sediment concentration, concurrently, simulation result indicates inter-particle force by particle collision is the key factor to reveal the sediment transport mechanism in the sheetflow layer, from the viewpoint of computational sediment transport.