Filter layers are extensively employed in dam structures as a mitigation method against internal erosion, which facilitates the interlocking and clogging of the suffused base soil due to its void geometry reduction. A series of tri-layer CFD-DEM models is established considering eight mono-and poly-dispersed filter materials to study the effect of particle size distribution (PSD) on its retention capability. The suffusion results, including the percentage of eroded mass (PME), soil spatial distribution and the velocity field, are analyzed in comparison of the 3D pore structure based on the maximal inscribed sphere (MIS) method. It is found that in contrast with the stable PME from lower soil, the internal erosion from the upper soil is mitigated to varying degrees with filter media of different PSDs from less than 5% of PME the filter of "seal with no erosion"to 19.8% in that of "large or continuous erosion". The analysis of Pore-PME correlation links the averaged MIS size with the suffusion results and provides reference for quantifying or estimating the mitigation effect of filter soil medium at a given PSD by evaluating its pore features based on the MIS method.