Disclosed is a method for preparing a cycloolefin polymer containing polar functional groups, comprising: preparing a catalyst mixture including i) a precatalyst, containing a Group 10 transition metal having a ligand containing oxygen ions bonded to the metal; ii) a first cocatalyst which is an organic compound containing a Group 15 element; and iii) a second cocatalyst which is capable of providing an anion and weakly coordinating to the metal of the precatalyst; and 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, at a temperature of 80-200.degree. C., the total amount of the organic solvent being 50-800% by weight based on the total weight of the monomer contained in the monomer solution, and the product yield of the polymer being 50% by weight or more based on the total weight of the monomer.