The fluid-solid mixture flow is the key phenomenon in the reactor core when severe accident happens, there are both fluid phase and solid phase in the migration of the molten corium. In this study, the discrete element method (DEM) is applied to model the interaction between solid bodies, the MPS method is adopted to simulate the fluid flows, and these two methods are coupled by the model of the solid and fluid phase interaction. The passively moving solid (PMS) model is adopted to modify the movement of moving particles so that several particles can form one solid body. The time step in the DEM model is different from that in the MPS method, thus a multiple time-step using inner iteration is carried out to solve this problem. The FOCUS (Fluid flow CoUpling with Solid's movement) code is developed by the models and methods above using C++ language. The simulations and experiments about collapse of solid cylinder layers and he breaking of a water dam involving solid cylinder layers in two-dimension are carried out to validate FOCUS, and a simple three-dimensional case is also tested before the future work about severe accidents in reactor cores. The comparisons between calculation results and experimental data are in good agreements with the average relative error less than 4%, meaning the FOCUS is capable of simulating the fluid-solid mixture flow. It provides the foundation for the further work on simulations of the severe accidents in nuclear reactor cores.