Solid oxide fuel cell (SOFC) is a clean and efficient power generation device, which has been widely used in recent years. However, its operation lifetime still needs to be improved to meet the requirements for further commercialization. The performance degradation of SOFCs is affected by many factors. It is very important to distinguish these factors and determine the dominant factor(s) for improving the operation lifetime in a more efficient manner. In this paper, the initial operation stage (<100 h) of SOFCs is mainly focused, in which the electrochemical performance changes most significantly. The electrochemical impedance spectroscopy (EIS) was periodically monitored during the operation process. Individual electrode processes of button cells and industrial-size cells were distinguished via distribution of relaxation time (DRT) method and subsequent equivalent circuit model (ECM) fitting. Through detailed time-varying analysis of EIS, the changes of different electrode processes in the initial-stage operation were obtained, and the evolution mechanism of electrochemical performance was proposed. After reduction, SOFCs go through activation stage and aging stage in turn: i) In the activation stage, the anode porosity increases and the gas-phase diffusion process is enhanced, resulting in the improvement of cell performance. ii) In the aging stage, the Ni particles in the anode agglomerate, so the effective three phase boundary (TPB) density decreases, which leads to the degradation of anode charge transfer reaction and the drop of cell performance. This evolution mechanism of electrochemical performance was partially confirmed by detailed microstructure characterization. It should be mentioned that the duration of each stage and the performance change in each stage can be affected by the initial electrode structure. Therefore, in previous studies, it was observed that some cells did not show activation process during initial-stage operation, but directly entered the aging stage. Besides, during longer-term operation of the cell (>100 h), we speculate that the dominant degradation mechanism is the continuous increase of the anode interface reaction-related resistance, which is caused by the agglomeration of Ni particles. However, since the anode structure is gradually stable, the degradation rate of cell performance decreases with respect to the operation time.