Single-atom catalysts (SACs) have been widely investigated and have emerged as a transformative approach in electrocatalysis. Despite their clear structure, the origin of their exceptional activity remains elusive. Herein, we elucidate a common phenomenon of the hybridization state transition of metal centers, which is responsible for the activity origin across various SACs for different reactions. Focusing on N-doped carbon-supported Ni SAC (NiN4 SAC) for CO2 reduction reaction (CO2RR), our comprehensive computations successfully clarify the hybridization state transition under working conditions and its relation with the activity. This transition, triggered by the reaction intermediates and applied potential, converts the Ni center from the inert dsp2 hybridization state to the active d2sp3 hybridization state. Importantly, the calculated activity and selectivity of the CO2RR over the d2sp3-hybridized Ni center are consistent with the experimental results, offering strong support for the proposed hypothesis. This work suggests a universal principle of electronic structure evolution in SACs that could revolutionize catalyst design, which also introduces a new paradigm for manipulating electronic states to enhance catalytic performance, with implications for various reactions and catalyst platforms.