The competitive adsorption of Cl– caused by anode OH– consumption under high current density is an important factor restricting the development of an alkaline seawater electrolyzer (ASWE). Here, we propose a strategy for rare earth corrosion protection which utilizes oxygen friendly rare earths that do not participate in the reaction to adsorb OH– and maintain the surface environment for stable anodic catalysis in an ASWE. Differential electrochemical mass spectrometry (DEMS) and electrochemical quartz crystal microbalance (EQCM) were used to identify the causes of chlorine corrosion on the high current anode plate of traditional Ni mesh. In situ fluorescence spectra of N-ethoxycarbonylmethyl-6-methoxyquinolinium bromide (MQAE) labeled with a chloride ion fluorescence probe, a rotating ring disk electrode (RRDE), and a time-resolved absorption spectrum were used to test the recognition mechanism of rare earth. Eu2O3 adsorbs OH– to maintain a high current pH environment and inhibits Cl– adsorption oxidation, thereby exhibiting stability for over 1000 h at 500 mA cm–2 current density. Furthermore, Eu2O3/FeNi2S4 was assembled into a kilowatt-level ASWE in 17 chambers with a total area of 1081.5 cm2 and operated stably for over 100 h at a current density of 500 mA cm–2 under industrial conditions of 80 °C and 30% KOH. Technical economic analysis (TEA) indicates that the rare earth corrosion protection strategy can enhance the service life of ASWE and reduce the cost of hydrogen production for profitable seawater hydrogen production, providing a new approach to solve the chlorine oxidation corrosion problem in an ASWE.