A catalyst composition for preparing a conjugated diene-based polymer, the 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 -N-(X 2 ) 3-a in Formula 1, a is an integer of 1 or 2, X 1 and X 2 are each independently selected from the group consisting of a hydrogen atom, monovalent C 1-20 hydrocarbon, - OR a , -SiR b R c R d and a covalent bonding functional group, or adjacent two functional groups in X 1 and X 2 are connected to form a heterocyclic compound, where at least one of X 1 and X 2 includes the covalent bonding functional group, where R a , R b , R c , and R d 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 -NR'R", 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, 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 -N-(X 2 ) 3-a in Formula 1, a is an integer of 1 or 2, X 1 and X 2 are each independently selected from the group consisting of a hydrogen atom, monovalent C 1-20 hydrocarbon, - OR a , -SiR b R c R d and a covalent bonding functional group, or adjacent two functional groups in X 1 and X 2 are connected to form a heterocyclic compound, where at least one of X 1 and X 2 includes the covalent bonding functional group, where R a , R b , R c , and R d 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 -NR'R", 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, and the covalent bonding functional group is a functional group containing a carbon-carbon double bond. a is an integer of 1 or 2, X 1 and X 2 are each independently selected from the group consisting of a hydrogen atom, monovalent C 1-20 hydrocarbon, - OR a , -SiR b R c R d and a covalent bonding functional group, or adjacent two functional groups in X 1 and X 2 are connected to form a heterocyclic compound, where at least one of X 1 and X 2 includes the covalent bonding functional group, where R a , R b , R c , and R d 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 -NR'R", 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, and the covalent bonding functional group is a functional group containing a carbon-carbon double bond. <EOS>