Disclosed is a method for preparing a thermoplastic resin with superior impact resistance, chemical resistance and processability as well as excellent gloss and whiteness. The method includes emulsion-polymerizing a conjugated diene compound monomer to prepare a rubber latex having an average particle diameter of 1,800 Å to 5,000 Å, a polymerization conversion ratio of at least 90% and a swelling index of 12 to 40, an average gel content of 70 to 95%, emulsion-polymerizing 45 to 75 parts by weight of the rubber latex with 17 to 40 parts by weight of an aromatic vinyl compound and 4 to 20 parts by weight of a vinyl cyanide compound to prepare a graft copolymer latex, and coagulating the graft copolymer latex with a coagulant at 60 to 80° C., and aging the graft copolymer latex at 80 to 99° C. to obtain a graft copolymer powder.