A method for preparing a graft copolymer includes the steps of: 1) preparing a seed by adding one or more selected from the group consisting of an alkyl (meth)acrylate-based monomer, an aromatic vinyl-based monomer, and a vinyl cyan-based monomer and carrying out polymerization; 2) preparing a core in the presence of the seed by adding an alkyl (meth)acrylate-based monomer and carrying out polymerization; and 3) preparing a shell in the presence of the core by adding an aromatic vinyl-based monomer and a vinyl cyan-based monomer and carrying out polymerization, wherein an activator including a compound represented by Chemical Formula 1 is added in the step 3). In Chemical Formula 1, R1 and R2 are each independently hydrogen, a C1-C10 alkyl group, or *-(C=O)OM2, provided that R1 and R2 are not both hydrogen, and
M1 and M2 are each independently an alkali metal.