The present invention provides a method for preparing a conjugated diene-based polymer including preparing a mixture of a molecular weight modifier and a conjugated diene-based monomer; and polymerization reacting the mixture using a catalyst composition including a lanthanide rare earth element-containing compound, modified methylaluminoxane, a halogen compound and an aliphatic hydrocarbon-based solvent, and therefore, capable of preparing a conjugated diene-based polymer having a high cis-1,4-bond content ratio, high linearity and narrow molecular weight distribution.