Method for preparing graft copolymer and method for preparing thermoplastic resin composition comprising same

The present disclosure relates to a method for preparing a graft copolymer and a method for preparing a thermoplastic resin composition comprising the prepared graft copolymer. More specifically, the present invention relates to a method for preparing a graft copolymer, the method comprising a process of adding an aromatic vinyl monomer and a vinyl cyanide monomer to a conjugated diene rubber latex amplified with a polymer particle size control agent and graft polymerizing the same, the polymer particle diameter control agent has a specific glass transition temperature and an average particle diameter at a specific ratio compared with the average particle diameter of the conjugated diene rubber latex; and a method for preparing a thermoplastic resin composition comprising the prepared graft copolymer. According to the present disclosure, a graft copolymer prepared using the conjugated diene rubber latex enlarged by the polymer particle size control agent shows a synergistic effect of improving both impact str