Method for preparing catalyst composition and method for preparing conjugated diene-based polymer

A method for preparing a catalyst composition, the method comprising:

• a step of alkylation for mixing and reacting a lanthanide rare earth element compound with an alkylating agent (S10);
• and
• a step of halogenation for mixing and reacting the lanthanide rare earth element compound alkylated in the step (S10), with a halogen compound (S20),
• wherein the step (S10) is performed by comprising a pre-treating agent, or
• the lanthanide rare earth element compound in the step (S10) is a lanthanide rare earth element compound of which hydrogen bond, oligomer type or a combination thereof is pre-treated by a pre-treatment step (S1) for preforming a pre-treatment reaction of the hydrogen bond, the oligomer type, or the combination thereof for the lanthanide rare earth element compound,
• the pre-treating agent is one or more trialkyl aluminums selected from the group consisting of tri-n-hexyl aluminum and tri-n-octyl aluminum, and
• the step (S1) is performed by mixing and reacting the lanthanide rare earth element compound, with one or more trialkyl aluminums selected from the group consisting of tri-n-hexyl aluminum and tri-n-octyl aluminum.