A catalyst composition for preparing a conjugated diene-based polymer, the catalyst composition comprising: a functionalizing agent of the following Formula 1; a rare earth metal compound; an alkylating agent; and a halogen compound: [Formula 1] (X 1) a -M 1 -([Y-M 2 -(X 2) n-1]) m-a in Formula 1, each of m and n is an integer of 3, a satisfies a relation of 0 ≤ a ≤ m-1, each of M 1 and M 2 is N, X 1 and X 2 are each independently selected from the group consisting of a hydrogen atom, monovalent C 1-20 hydrocarbon, - OR a, -NR b R c, -SiR d R e R f and a covalent bonding functional group, where R a, R b, R c, R d, R e and R f are each independently selected from the group consisting of a hydrogen atom, C 1-20 alkyl, C 3-20 cycloalkyl, C 6-20 aryl, C 7-20 alkylaryl, C 7-20 arylalkyl, -NR'R" and a covalent bonding functional group, where R' and R" are each independently selected from the group consisting of a hydrogen atom, C 1-20 alkyl, C 3-20 cycloalkyl, C 6-20 aryl, C 7-20 alkylaryl, C 7-20 arylalkyl, and a covalent bonding functional group, Y is a divalent hydrocarbon group substituted with at least one covalent bonding functional group, and the covalent bonding functional group is a functional group containing a carbon-carbon double bond. a functionalizing agent of the following Formula 1; a rare earth metal compound; an alkylating agent; and a halogen compound: [Formula 1] (X 1) a -M 1 -([Y-M 2 -(X 2) n-1]) m-a in Formula 1, each of m and n is an integer of 3, a satisfies a relation of 0 ≤ a ≤ m-1, each of M 1 and M 2 is N, X 1 and X 2 are each independently selected from the group consisting of a hydrogen atom, monovalent C 1-20 hydrocarbon, - OR a, -NR b R c, -SiR d R e R f and a covalent bonding functional group, where R a, R b, R c, R d, R e and R f are each independently selected from the group consisting of a hydrogen atom, C 1-20 alkyl, C 3-20 cycloalkyl, C 6-20 aryl, C 7-20 alkylaryl, C 7-20 arylalkyl, -NR'R" and a covalent bonding functional group, where R' and R" are each independently selected from the group consisting of a hydrogen atom, C 1-20 alkyl, C 3-20 cycloalkyl, C 6-20 aryl, C 7-20 alkylaryl, C 7-20 arylalkyl, and a covalent bonding functional group, Y is a divalent hydrocarbon group substituted with at least one covalent bonding functional group, and the covalent bonding functional group is a functional group containing a carbon-carbon double bond. each of m and n is an integer of 3, a satisfies a relation of 0 ≤ a ≤ m-1, each of M 1 and M 2 is N, X 1 and X 2 are each independently selected from the group consisting of a hydrogen atom, monovalent C 1-20 hydrocarbon, - OR a, -NR b R c, -SiR d R e R f and a covalent bonding functional group, where R a, R b, R c, R d, R e and R f are each independently selected from the group consisting of a hydrogen atom, C 1-20 alkyl, C 3-20 cycloalkyl, C 6-20 aryl, C 7-20 alkylaryl, C 7-20 arylalkyl, -NR'R" and a covalent bonding functional group, where R' and R" are each independently selected from the group consisting of a hydrogen atom, C 1-20 alkyl, C 3-20 cycloalkyl, C 6-20 aryl, C 7-20 alkylaryl, C 7-20 arylalkyl, and a covalent bonding functional group, Y is a divalent hydrocarbon group substituted with at least one covalent bonding functional group, and the covalent bonding functional group is a functional group containing a carbon-carbon double bond.