Abstract: In developing gas filled hohlra um targets, fabrication of polyimide with thickness no more than 1m m plays an important role, and its kernel technology is synthetic polypyromellitamic acid. Th e solution viscosity is measured during the synthetic process by the method of supersonic wave. Preliminary experiments have been performed to determine the effects of reaction conditions in order to get high molecular weight in the solution. The best synthetic condition is concluded as follows: solid pyromellitic dianhydride should be added to diamine solutions, mole ratio of dianhydride and diamine should be 1.01～1.02∶1, the reaction temperature should be 0～5℃, water in solvent should be least and reaction should be stopped in suitable time.