Abstract: The infiltration and subsequent degradation induced by molten calcium-magnesium-alumina-silicate (CMAS) at high service temperatures remain a critical challenge triggering premature failure of thermal barrier coatings (TBCs) in advanced aero-engines. Consequently, tailoring coating microstructures to physically block CMAS intrusion has emerged as a core strategy to boost coating durability. Focusing on the Porous Embedded Particle Clusters (PEPC) structure, which possesses outstanding CMAS resistance potential, this study systematically investigates the infiltration dynamics of molten CMAS. A coupled numerical model integrating two-dimensional creeping flow and particle tracing is established to reveal how geometric features of particles inside flat spindle-shaped pores influence CMAS infiltration and the corresponding underlying mechanisms. Simulation results show that at the dual-particle scale, the synergistic matching of large particles and micro-gaps greatly strengthens the throttling effect of pore throats. Meanwhile, arranging particles perpendicular to the flow direction effectively suppresses the hydrodynamic shielding effect and maximizes barrier efficiency. At the multi-particle scale, average particle spacing is proven to be the primary factor dominating anti-CMAS infiltration capacity. A nonlinear threshold control mechanism is uncovered, identifying a critical spacing of roughly 0.7 μm for PEPC structures with 72% porosity; below this threshold, infiltration resistance rises sharply. Furthermore, this work elaborates a strong synergistic barrier mechanism coupling particle spacing, porosity and pore tortuosity. It is demonstrated that raising the fraction of fine particles produces a dual barrier effect by simultaneously narrowing average particle spacing and increasing flow channel tortuosity. Ultimately, these results reveal the physical nature of infiltration suppression in PEPC architectures, laying a solid theoretical basis for microstructure optimization of next-generation high-performance TBCs.