Abstract: ODPA was used to replace BTDA in PMR-15, and PMDA was added into dianhydride. ODPA/PMDA-MDA PMR copolyimide resin system was synthesized with copolymerization. The resin-solution's viscosity, the processing property, the thermal-oxidation stability and flexural property of cured resins were characterized. The relationship between resin structure and the properties of the resin was also researched. The experimental results show that resin-solution's viscosity increases with increasing molecular weight and decreases with increasing PMDA content. T_g of resin decreases with increasing molecular weight and increases with increasing PMDA content. Softening temperature of resin increases with increasing molecular weight and PMDA content. The temperature of resin weight loss and flexural property change complexly.