The present invention relates to a method for preparing an acrylonitrile-butadiene-styrene graft copolymer having improved impact strength, which comprises: a step of preparing diene-based rubber latex by adding an acrylate-based cross-linking agent (Step 1); and a step of inserting the same cross-linking agent with the acrylate-based cross-linking agent, an aromatic vinyl-based compound and a vinyl cyan-based compound to the diene-based rubber latex and then subjecting thereof to graft copolymerization (Step 2), an acrylonitrile-butadiene-styrene graft copolymer prepared by the method, and an acrylonitrile-butadiene-styrene thermoplastic resin comprising the same.