Disclosed is a method of preparing a rubber-reinforced thermoplastic resin. According to the present invention, a graft copolymer comprising a large-particle-size rubber polymer obtained by welding polymerization-heat-control-type small particles and a reactive emulsifier or a mixed emulsifier comprising a reactive emulsifier and a non-reactive emulsifier is prepared, and a rubber-reinforced thermoplastic resin comprising the same may have enhanced mechanical properties, thermal stability, surface visibility and gloss.