Abstract: Elevated-temperature operation has attracted increasing attention in the development of proton exchange membrane fuel cells(PEMFCs). Under elevated temperatures and gas pressures, the cathode gas diffusion layer(GDL) is exposed to significantly harsher conditions. The degradation of the cathode GDL under elevated-temperature operation is investigated via elevated-temperature durability testing, characterization and theoretical analysis. The results indicate that the mass transport resistance of the fuel cell at high current densities increased to four times that of the fresh fuel cell after elevated-temperature operation. Furthermore, severe degradation of the cathode GDL is observed, primarily due to the detachment of carbon fillers between fibers, carbon corrosion, and the loss of hydrophobic agents. The loss of carbon fillers during operation alters the internal structure of cathode GDL, reducing compressive strength, while carbon corrosion decreases the filler modulus. The internal structure of the cathode GDL changes due to the carbon fillers being washed off during elevated-temperature operation. This results in a 25.37% decrease in compression strength and a 53.82% decrease in the modulus of the carbon fillers due to carbon corrosion. The movement of carbon fillers enlarges macropores while reducing meso- and micropores, thereby impeding effective mass transport. A 25.86% reduction in peak MEA power density is quantitatively revealed by the reassembled cathode GDL experiment, caused by elevated-temperature degradation. This work offers valuable insights into the degradation mechanisms of cathode GDL under elevated-temperature operation, supporting future efforts in PEMFC design and durability enhancement.