The aged stack results in resistance growth and power decline. At present, most of the analyses of resistance growth are qualitative or identified by complex mechanism models. For more effective identification, the distribution of relaxation times (DRT) method is applied to the aging analysis of the stack. The individual polarization process of the stack corresponding to each DRT peak is determined by appropriate experimental conditions and the impedance of the individual polarization process is characterized by the peak area. The three DRT peaks from low frequency to high frequency are identified as the mass transport, the charge transfer of oxygen reduction reactions (ORRs), and the proton transport in the cathode catalyst layer (CCL) and anode side. The stack’s voltage recession rate is 15% at the rated current density of 800 mA cm⁻² after running for 2000 h in the driving cycle. Mass transport is the main reason accounting for 66.1% of the resistance growth. The charge transfer resistance growth cannot be ignored, accounting for 30.23%. The resistance growth obtained by the DRT can quickly and accurately identify the main reason for stack decline and therefore promises to become an important diagnostic tool in relation to aging.