Abstract: To investigate the degradation behavior of proton exchange membrane fuel cells (PEMFCs) under dynamic conditions of locomotives, we have conducted a 900₋hour durability test on a 300 W stack. A novel approach combining the distribution of relaxation time and the transmission line model is proposed to evaluate the degradation mechanism. The single cell with significant voltage drop is selected for the study according to the inconsistency analysis of the stack. Based on the polarization curve and impedance spectroscopy analysis, the distribution of relaxation time is used to investigate the trend of polarization processes in each frequency band, and the transmission line model is established for parameter identification and quantification. Finally, scanning electron microscope (SEM) is used to characterize the microscopic morphology of different cells. The experimental results show that during 0~500 h period, the catalyst performance remains essentially stable, while carbon corrosion increases electrode porosity and alleviates the mass transport loss; during 500~900 h period, the performance of catalyst accelerates decay. Carbon corrosion accumulates and causes severe damage to the electrode structure. The loss of numerous micropores and subsequent structural compaction and collapse rapidly deteriorate the mass transport performance, ultimately resulting in the failure of PEMFC. The results show that the proposed method can effectively evaluate the degradation mechanism of porous electrode in fuel cells.