Disclosed is a method for preparing a ternary alloy catalyst with polydopamine coating and a ternary alloy catalyst prepared thereby. The method for preparing a ternary alloy catalyst according to the present disclosure may provide a ternary alloy catalyst with increased resistance to carbon monoxide (CO) poisoning in which polydopamine is utilized as a coating material for a ternary alloy catalyst having a core-shell structure containing platinum to suppress the growth of particles during subsequent high-temperature heat treatment, and nickel (Ni), which is a transition metal, is diffused inside to form a core, thereby effectively preventing elution of nickel under an acidic condition.