The present invention relates to a method of preparing a copolymer having controlled gel content by polymerizing cyclic olefin compound by ROMP (Ring Opening Metathesis Polymerization), and more precisely, a method of preparing cyclic olefin copolymer with high yield, in which a complex prepared by mixing a W salt compound and an organoaluminum halide compound is used as a catalyst in the presence or absence of a molecular weight regulator, and a method of controlling gel formation by regulating the composition of cyclic olefin monomer composing a copolymer as shown in the following Empirical Formula 1 without using a reaction speed regulator or gel formation inhibitor. (Empirical Formula 1) Norbornene=30.about.70 weight % Cyclic olefin compound=70.about.30 weight % At this time, the cyclic olefin compound presented in the above Empirical Formula 1 is one or more compounds selected from a group consisting of dicyclopentadiene and compounds represented by the following Formula 1. ##STR00001## Wherein, m is an integer of 0-1, R.sub.1 is C.sub.1-C.sub.5 alkyl and R.sub.2 is H or C.sub.1-C.sub.10 alkyl.