Provided are a method of preparing a graft copolymer, a graft copolymer, and a thermoplastic resin molded article including the graft copolymer, the method including: 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 performing polymerization; 2) in the presence of the seed, preparing a core by adding an alkyl (meth)acrylate-based monomer and performing polymerization; and 3) in the presence of the core, preparing a shell by adding an aromatic vinyl-based monomer and a vinyl cyan-based monomer and performing polymerization, wherein the method further includes adding an alkyl acrylate-based polymer. When the alkyl acrylate-based polymer is included in a graft copolymer, a graft copolymer and a thermoplastic resin molded article excellent in weather resistance, fluidity, mechanical properties, and appearance quality can be provided.