Abstract: In high-temperature water electrolysis using Solid Oxide Electrolysis Cells, the charge transport resistance and reaction load distribution imbalance caused by micro-topological structures are critical bottlenecks limiting high-intensity operation. This study aims to quantitatively reveal the influence of porosity and pore-size gradient designs on electrochemical polarization behavior via mesoscale heterogeneous simulation, exploring structural refinement criteria to balance transport and electrochemical activity. Three structures were generated based on a digital reconstruction algorithm: a uniform electrode (Case A), a single-porosity gradient electrode (Case B), and a porosity/pore-size dual-gradient electrode (Case C). Under the condition of a constant total porosity of 0.33 and uniform thickness across all structures, a mesoscale multiphysics coupled heterogeneous model was established. The simulation results reveal the evolution mechanism of microstructure-induced reaction characteristics with current density. Under a light load of 3 000 A/m², the uniform Case A performs best, with a maximum activation overpotential ( \eta _\textact ) of only 0.087 V, while the \eta _\textact of the single-gradient Case B reaches 0.117 V due to the deficiency of active sites near the electrolyte side. At a load of 8 000 A/m², the gas-phase concentration remains sufficient across all structures, indicating that gas-phase mass transfer is not the primary limiting factor at this stage; however, electrode performance changes significantly: the \eta _\textact of the uniform structure increases to 0.152 V. Although the dual-gradient structure exhibits a slightly higher \eta _\textact (0.097 V) than the uniform structure at 3 000 A/m² due to its structural complexity, it demonstrates superior anti-polarization capability at high load, with an \eta _\textact of only 0.147 V, which is lower than that of the uniform structure. Research indicates that the uniform structure experiences increased polarization loss at high current due to the elevated load on the triple-phase boundaries (TPBs) near the electrolyte side. Due to a greater deficiency of active sites compared to the uniform structure, the single-gradient structure suffers from an over-concentration of the reaction load, resulting in the worst performance across the entire current range. The dual-gradient design utilizes large pores near the gas channel side to reduce transport tortuosity and construct an efficient transport network, while significantly enhancing TPB density (3.063 μm⁻²) by refining the pore size to effectively disperse local reaction stress near the electrolyte side. Therefore, the dual-gradient structure is better suited for large-scale SOEC applications. The proposed mesoscale heterogeneous evaluation method provides a scientific basis and quantitative guidance for the structural design of high-performance SOEC cathodes.