Provided are a method for preparing a conjugated diene-based polymer and a method for preparing a graft copolymer including the same. The method for preparing a conjugated diene-based polymer includes enlarging a first conjugated diene-based polymer in multiple stages to prepare a second conjugated diene-based polymer, wherein the multi-stage enlargement includes a primary enlargement stage and a secondary enlargement stage, and a weight ratio of acids added in the primary enlargement stage and the secondary enlargement stage is 70:30 to 90:10. A graft copolymer prepared by the above-described method can be used to form a thermoplastic resin molded article excellent in all of plating characteristics, mechanical properties, surface characteristics, colorability, and processability.