Disclosed is a method for preparing a cycloolefin polymer containing polar functional groups, comprising the steps of: a) preparing a catalyst mixture including i) a precatalyst; ii) a first cocatalyst; and iii) a second cocatalyst; and b) subjecting a monomer solution comprising a norbornene-based compound containing a polar functional group to an addition polymerization reaction in the presence of an organic solvent and the catalyst mixture, wherein the product yield of the prepared polymer is 50% by weight or more based on the total weight of the monomer.