Provided is a method of preparing a graft copolymer, which includes: preparing a liquid maleimide-based monomer; preparing a reaction solution including a diene-based rubber polymer, an aromatic vinyl-based monomer, and a vinyl cyan-based monomer; and adding the liquid maleimide-based monomer and the reaction solution to a reactor and carrying out polymerization. According to the preparation method of the present invention, the amount of the maleimide-based monomer involved in the polymerization can be increased, and the color characteristics, glass transition temperature, and softening temperature of the graft copolymer are improved.