Detailed explanation and reliable quantification of the numerous degradation mechanisms contained in solid oxide fuel cells (SOFCs) are key issues to improve their durability. Although electrochemical impedance spectroscopy (EIS) has been widely used to unfold the complex and coupled physical/chemical processes, there are still some concerns with respect to the measurement and analysis procedures. In this study, an industrial-size cell (10 x 10 cm(2)) is tested to clarify the evolution of electrochemical characteristics during initial-stage operation, including 5 h of anode reduction, 32 h of activation process and 40 h of initial aging process. Detailed analysis of EIS measured under different DC bias is implemented through distribution of relaxation times (DRT) and subsequent equivalent circuit model (ECM) fitting to identify the contributions of individual processes to the rapid performance degradation during initial aging process. It is found that the deterioration of anode charge transfer reactions and ionic transport jointly causes more than 60% of the voltage degradation, followed by the O-2 surface exchange kinetics coupled with O2- diffusion (17.3%), and then the anode gas conversion (13%). The microstructure deterioration of anode/electrolyte interface caused by Ni redistribution is regarded as the dominant degradation mechanism during initial aging process. A fast Ni migration mechanism is proposed to explain the observable Ni depletion in the anode functional layer, which is verified by detailed post-test characterization.