By using bicyclo [2.2.1] hept-5-ene-2，3-diearboxylie acid ethyl ester，benzonphe-nonetetracarboxylic acid diethyl ester and methylene dianiline as monomers and ethanol as solvent“PMR”polyimide was synthesized．This type of pvlyimide is suitable for preparation of composites．The results of isothermal aging at 300℃ indicated that the glass cloth reinforced plastic of this polyimide can be used in the range of 250一300℃ for long period．It has been verified that the ester．ammonium salt was formed in the resin solution as soon as the monomers were mixed．The separation of the resin solution in ethanol into two layers was overcome．The reaction mechanism of“PMR”polyimide，such as amidation，imidation and crosslinking．were studied．