A method of preparing a graft copolymer includes a step of performing coagulation by adding an acid coagulant to 100 parts by weight (based on solids) of graft copolymer latex prepared by graft-polymerizing an aromatic vinyl compound and a vinyl cyanide compound onto conjugated diene rubber latex; and a step of performing coagulation once more by adding a salt coagulant thereto. In a resulting graft copolymer, coagulation efficiency may be improved and the gloss of the prepared graft copolymer may be increased. In addition, since a b-value measured using a Hunter lab colorimeter and a b-value measured after being left at 250° C. for 15 minutes are low, the graft copolymer may have excellent color characteristics. In addition, since heating loss is low and scorch time is long, the graft copolymer may have excellent processing characteristics.