A method of preparing a conjugated diene-based polymer is provided. The method includes: adding a conjugated diene-based monomer, a catalyst, and a solvent to parallel polymerization reactors, performing a polymerization reaction to prepare a first polymer solution including a first conjugated diene-based polymer; and adding the first polymer solution to a polymerization reactor connected in series to the parallel polymerization reactors, performing the polymerization reaction to prepare a second polymer solution including a second conjugated diene-based polymer. After the polymerization reaction in the series polymerization reactor, gas produced by polymerization heat is condensed in the series polymerization reactor and refluxed to the parallel polymerization reactors, and a reaction temperature of the series polymerization reactor is maintained to be lower than a reaction temperature of the parallel polymerization reactors by 10° C. or more.