Provided is a novel fluorene-based polymer having urethane bonds and a method for preparing the fluorene-based polymer. According to the method, a diol compound is condensed with a diisocyanate instead of an acid dianhydride, and then an acid anhydride is reacted with the reaction mixture to introduce desired acid groups into the final polymer. Further provided is a negative-type photosensitive resin composition comprising of the fluorene-based polymer as a binder resin. The composition exhibits improved chemical resistance, good development margin and high sensitivity due to presence of the acid groups despite the low molecular weight of the fluorene-based polymer.