A method for preparing a modified conjugated diene-based polymer, the method comprising: 1) polymerizing a conjugated diene-based monomer in the presence of a catalyst composition in a hydrocarbon solvent to prepare an active polymer; 2) reacting the active polymer with a modifier represented by the following Formula 1 to prepare a first polymer; and 3) adding a sulfur halide to the first polymer and mixing for 15 minutes or more: in Formula 1, R 1 to R 3 are each independently a trivalent hydrocarbon group substituted with one or more substituents selected from the group consisting of a halogen group, an alkyl group of 1 to 20 carbon atoms, a cycloalkyl group of 3 to 20 carbon atoms, an aryl group of 6 to 30 carbon atoms, and -R 6 COOR 7; or an unsubstituted divalent hydrocarbon group of 1 to 10 carbon atoms, where all R 1 to R 3 are not trivalent hydrocarbon groups; or divalent hydrocarbon groups at the same time, R 4 is a single bond, an alkylene group of 1 to 20 carbon atoms, or a cycloalkylene group of 3 to 20 carbon atoms, R 5 is a silyl group that is unsubstituted or substituted with an alkyl group of 1 to 20 carbon atoms; halogen; a cyano group; or -COR 8, R 6 is a single bond, an alkylene group of 1 to 20 carbon atoms, or a cycloalkylene group of 3 to 20 carbon atoms, R 7 is an alkyl group of 1 to 20 carbon atoms, or a cycloalkyl group of 3 to 20 carbon atoms, and R 8 is one selected from the group consisting of an alkoxy group of 1 to 10 carbon atoms, an aryl group of 6 to 30 carbon atoms, a heteroaryl group of 2 to 30 carbon atoms, a heterocycloalkyl group of 2 to 10 carbon atoms, a heteroamine group of 2 to 10 carbon atoms and a disilylamino group of 3 to 10 carbon atoms. 1) polymerizing a conjugated diene-based monomer in the presence of a catalyst composition in a hydrocarbon solvent to prepare an active polymer; 2) reacting the active polymer with a modifier represented by the following Formula 1 to prepare a first polymer; and 3) adding a sulfur halide to the first polymer and mixing for 15 minutes or more: in Formula 1, R 1 to R 3 are each independently a trivalent hydrocarbon group substituted with one or more substituents selected from the group consisting of a halogen group, an alkyl group of 1 to 20 carbon atoms, a cycloalkyl group of 3 to 20 carbon atoms, an aryl group of 6 to 30 carbon atoms, and -R 6 COOR 7; or an unsubstituted divalent hydrocarbon group of 1 to 10 carbon atoms, where all R 1 to R 3 are not trivalent hydrocarbon groups; or divalent hydrocarbon groups at the same time, R 4 is a single bond, an alkylene group of 1 to 20 carbon atoms, or a cycloalkylene group of 3 to 20 carbon atoms, R 5 is a silyl group that is unsubstituted or substituted with an alkyl group of 1 to 20 carbon atoms; halogen; a cyano group; or -COR 8, R 6 is a single bond, an alkylene group of 1 to 20 carbon atoms, or a cycloalkylene group of 3 to 20 carbon atoms, R 7 is an alkyl group of 1 to 20 carbon atoms, or a cycloalkyl group of 3 to 20 carbon atoms, and R 8 is one selected from the group consisting of an alkoxy group of 1 to 10 carbon atoms, an aryl group of 6 to 30 carbon atoms, a heteroaryl group of 2 to 30 carbon atoms, a heterocycloalkyl group of 2 to 10 carbon atoms, a heteroamine group of 2 to 10 carbon atoms and a disilylamino group of 3 to 10 carbon atoms. R 1 to R 3 are each independently a trivalent hydrocarbon group substituted with one or more substituents selected from the group consisting of a halogen group, an alkyl group of 1 to 20 carbon atoms, a cycloalkyl group of 3 to 20 carbon atoms, an aryl group of 6 to 30 carbon atoms, and -R 6 COOR 7; or an unsubstituted divalent hydrocarbon group of 1 to 10 carbon atoms, where all R 1 to R 3 are not trivalent hydrocarbon groups; or divalent hydrocarbon groups at the same time, R 4 is a single bond, an alkylene group of 1 to 20 carbon atoms, or a cycloalkylene group of 3 to 20 carbon atoms, R 5 is a silyl group that is unsubstituted or substituted with an alkyl group of 1 to 20 carbon atoms; halogen; a cyano group; or -COR 8, R 6 is a single bond, an alkylene group of 1 to 20 carbon atoms, or a cycloalkylene group of 3 to 20 carbon atoms, R 7 is an alkyl group of 1 to 20 carbon atoms, or a cycloalkyl group of 3 to 20 carbon atoms, and R 8 is one selected from the group consisting of an alkoxy group of 1 to 10 carbon atoms, an aryl group of 6 to 30 carbon atoms, a heteroaryl group of 2 to 30 carbon atoms, a heterocycloalkyl group of 2 to 10 carbon atoms, a heteroamine group of 2 to 10 carbon atoms and a disilylamino group of 3 to 10 carbon atoms.