In situ detection of highly‐oxidized metal intermediates is the key to identifying the active center of an oxygen evolution reaction (OER) catalyst, but it remains challenging for NiFe‐based catalysts in an aqueous solution under working conditions. Here, by utilizing the dynamic stability of the FeVIO42− intermediates in a self‐healing water oxidation cycle of NiFe‐based catalyst, the highly‐oxidized FeVI intermediates leached into the electrolyte are directly detected by simple spectroelectrochemistry. Our results provide direct evidence that Fe is the active center in NiFe‐based OER catalysts. Furthermore, it is revealed that the incorporation of Co into NiFe‐based catalyst facilitates the formation of FeVI active species, thus enhancing the OER activity of NiCoFe‐based catalyst. The insights into the mechanisms for the sustainable generation of FeVI active species in these NiFe‐based catalysts lay the foundation for the design of more efficient and stable OER catalysts.