Abstract: Provided is a method for preparing an alloy catalyst for fuel cells having excellent catalytic activity and high durability. The method includes coating a platinum or platinum-transition metal catalyst supported on carbon with polydopamine as a capping agent. The method for preparing an alloy catalyst supported on carbon uses polydopamine as a capping agent for a platinum or platinum-transition metal catalyst supported on carbon, and thus provides a binary or ternary platinum alloy catalyst supported on carbon having a small particle size and high alloying degree despite the subsequent high-temperature heat treatment. In addition, polydopamine (PDA) is a highly adhesive material and allows thin and uniform coating, and thus inhibits particle size growth during heat treatment while allowing easy diffusion of a transition metal into the metal.