The present invention provides a method of preparing a catalyst for PEM water electrolysis, which includes dispersing boron carbide (B4C) in water to prepare an aqueous boron carbide solution; adding an iridium (Ir) precursor to the aqueous boron carbide solution to prepare a mixed solution; adjusting acidity of the mixed solution to be in a basic region; and adding a reducing agent to the mixed solution and stirring the mixture to form Ir/B4C composite nanoparticles. The catalyst for PEM water electrolysis prepared by the method has high durability and a large specific surface area.