A blocked diisocyanate polyester was synthesized to copolymerize with novolak-type phenolic resin. From the results of TR and NMR spectra, it was found that the blocked diisocyanate polyester reacts with the hydroxyl group of the novolak-type phenolic resin at high temperature.The deblocking temperature for the novolak-type phenolic resin is 120 degrees C. The blocking agent, epsilon-caprolactam, is convenient for this system. The copolymer exhibited a single glass transition temperature and a negative deviation of the glass transition temperature. The results showed that the copolymer system is miscible and the molecular motion of the copolymer is increased with the addition of a modifier. The interface between the modified phenolic resin and the glass fiber is improved and the tensile strength of pultruded composite is increased with the modifier content.