A method of preparing an ABS graft copolymer, comprising: A) a first polymerization step of feeding 50 to 70 parts by weight of a conjugated diene rubber latex, 5 to 15 parts by weight of an aromatic vinyl compound, 1 to 10 parts by weight of a vinyl cyanide compound, 0.05 to 1 part by weight of an emulsifier, 0.01 to 0.5 parts by weight of an initiator, and 0.001 to 0.5 parts by weight of an oxidation-reduction catalyst into a reactor batchwise, based on 100 parts by weight of a total composition containing the conjugated diene rubber latex, the aromatic vinyl compound, and the vinyl cyanide compound, and performing graft polymerization; B) a second polymerization step of performing graft polymerization while continuously feeding an emulsion containing 15 to 30 parts by weight of the aromatic vinyl compound, 5 to 15 parts by weight of the vinyl cyanide compound, 0.05 to 0.5 parts by weight of a molecular weight modifier, and 0.05 to 0.5 parts by weight of the initiator based on 100 parts by weight of the total composition when a polymerization conversion rate reaches 5 to 40 %; C) a third polymerization step of performing graft polymerization while continuously feeding 0.05 to 1 part by weight of a phosphate ester-based reactive emulsifier based on 100 parts by weight of the total composition when a polymerization conversion rate reaches 75 to 95 %, and D) a fourth polymerization step, in which, after initiation or completion of the third polymerization step, 0.01 to 0.2 parts by weight of the initiator and 0.001 to 0.3 parts by weight of the oxidation-reduction catalyst are additionally fed into the reactor, and graft polymerization is performed, wherein the phosphate ester-based reactive emulsifier is a compound represented by Formula 1 below: wherein R 1 is an alkylene having 1 to 10 carbon atoms, R 2 is a linear or branched alkylene having 3 to 10 carbon atoms, x is an integer of 1 to 4, and y is an integer of 1 to 3. A) a first polymerization step of feeding 50 to 70 parts by weight of a conjugated diene rubber latex, 5 to 15 parts by weight of an aromatic vinyl compound, 1 to 10 parts by weight of a vinyl cyanide compound, 0.05 to 1 part by weight of an emulsifier, 0.01 to 0.5 parts by weight of an initiator, and 0.001 to 0.5 parts by weight of an oxidation-reduction catalyst into a reactor batchwise, based on 100 parts by weight of a total composition containing the conjugated diene rubber latex, the aromatic vinyl compound, and the vinyl cyanide compound, and performing graft polymerization; B) a second polymerization step of performing graft polymerization while continuously feeding an emulsion containing 15 to 30 parts by weight of the aromatic vinyl compound, 5 to 15 parts by weight of the vinyl cyanide compound, 0.05 to 0.5 parts by weight of a molecular weight modifier, and 0.05 to 0.5 parts by weight of the initiator based on 100 parts by weight of the total composition when a polymerization conversion rate reaches 5 to 40 %; C) a third polymerization step of performing graft polymerization while continuously feeding 0.05 to 1 part by weight of a phosphate ester-based reactive emulsifier based on 100 parts by weight of the total composition when a polymerization conversion rate reaches 75 to 95 %, and D) a fourth polymerization step, in which, after initiation or completion of the third polymerization step, 0.01 to 0.2 parts by weight of the initiator and 0.001 to 0.3 parts by weight of the oxidation-reduction catalyst are additionally fed into the reactor, and graft polymerization is performed, wherein the phosphate ester-based reactive emulsifier is a compound represented by Formula 1 below: wherein R 1 is an alkylene having 1 to 10 carbon atoms, R 2 is a linear or branched alkylene having 3 to 10 carbon atoms, x is an integer of 1 to 4, and y is an integer of 1 to 3. wherein the phosphate ester-based reactive emulsifier is a compound represented by Formula 1 below: wherein R 1 is an alkylene having 1 to 10 carbon atoms, R 2 is a linear or branched alkylene having 3 to 10 carbon atoms, x is an integer of 1 to 4, and y is an integer of 1 to 3.