The fuel basket for spent fuel storage and transportation (basket) is a crucial equipment for storing and transporting spent fuel. The structural integrity of the basket under seismic conditions is vital to the safety of spent fuel storage. The basket features a complex internal structure with dense fluid channels, making it a porous structure (hereinafter referred to as the porous basket). Under seismic conditions, the porous basket exhibits intense fluid–structure interaction (FSI) with the water in the spent fuel pool. However, the mechanism of the FSI effect of this porous structure on the seismic resistance of the basket remains unclear. Currently, the calculation of FSI for such porous structures primarily relies on detailed Computational Fluid Dynamics (CFD) modeling. However, due to the significant difference in scale between the pores within the basket and the basket itself, this method is computationally expensive. Therefore, it is necessary to establish a macro-scale model to describe the oscillatory flow within the porous structure. To address this issue, this paper first utilizes OpenFOAM to conduct Direct Numerical Simulation (DNS) and establish a fluid model at the pore scale. Based on the simulation results, an equivalent model for oscillatory flow within the porous structure at the macro scale is then constructed. Finally, using this model, the FSI of a single basket vibrating in water is studied, yielding the added mass and added damping. This research is of great significance for the seismic safety assessment of the grid.