Belonging to the technical field of electrocatalyst synthesis, the invention relates to an iron-based catalyst, a preparation method and application thereof in efficient electrocatalytic water splitting. The preparation method includes: firstly synthesizing FeS nanosheet by solvothermal method, and then conducting electrochemical in situ activation to obtain an Fe@FeOxSy core-shell nanoparticle iron-based catalyst. The product of the invention has excellent electrocatalytic water splitting activity and stability under an alkaline condition; electrocatalytic water splitting hydrogen evolution only needs overpotential of 510mV, and can reach an industrial grade standard level current density of 1000mA/cm<-2>; electrocatalytic water splitting oxygen evolution only needs overpotential of 240mV, and can reach a current density of 10mA/cm<-2> required by composing an alkaline electrolytic tank, the iron-based catalyst provided by the invention is far better than the current industrial noble metal catalysts, and has stability up to at least 1000h without performance attenuation. The preparation method has the advantages of simplicity, convenience and controllability, short preparation period and easy large-scale preparation. The needed raw materials have abundant reserves, are low in cost, and can replace noble metals and promote commercial application of hydrogen production by water splitting.