The present invention provides a method of preparing a graft copolymer, which comprises graft emulsion polymerization of a diene-based rubber polymer with an aromatic vinyl-based monomer and a vinyl cyanide-based monomer in the presence of an azo-based initiator having a solubility in water at 20° C. of 10 g/100 g or more and a 10-hour half-life decomposition temperature in water of 80° C. or less, wherein the graft emulsion polymerization is initiated at 45 to 55° C. According to the present invention, a method of preparing a graft copolymer, which allows polymerization to be performed at low temperatures and ensures excellent polymerization stability, can be provided.