<p>High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) offer promising power sources for heavy-duty vehicles (HDV) due to their effective management of heat and water. However, the long-term stability of HT-PEMFCs is critically constrained by phosphoric acid (PA) electrolyte loss especially at high current working circumstances. Herein this work, gas diffusion layers (GDLs) of distinct porous structures were designed to regulate PA distribution under rigorous loading circumstance. Combined with high-current–density (HCD) accelerated stress tests (AST) at 1.6 A cm⁻² and in-operando 3D X-ray microscopic (XRM) characterization to systematically elucidate the influence of GDL structure on acid-loss pathways and performance degradation, the GDL featuring a dense microporous layer (MPL) with low mesoporosity exhibited the superior anti-leaching capability, showing the lowest acid loss rate (2.1&#xa0;ng&#xa0;cm⁻²&#xa0;h⁻¹ L⁻¹) and the smallest voltage decay (0.23&#xa0;mV cycle⁻¹) during HCD AST. Mechanistic analysis reveals that the dense MPL effectively mitigates liquid PA migration through catalyst layer (CL) cracks, while the high porosity compensates the performance loss led by mass transport resistance. This work suggests that engineering the overall GDL structure is an effective route to achieving long-term stable HT-PEMFC operation at HCD, providing key theoretical guidance for the design of next-generation, high-performance and durable membrane electrode assemblies (MEAs) for the heavy-duty applications.</p>