Abstract: A polyamide acid oligomer (PAA-n), which was mainly consist of PAA trimer, was synthesized by using 3,3′,4,4′-diphenylether tetracarboxylic acid dianhydride and 4,4′- diamino diphenyl ether as starting materials. The structure and properties of the product were characterized by FT-IR, DSC and ESI. Thermo and mechanical performance of modified epoxy resin system were studied by using PAA-n to modify epoxy resin. Results showed that the synthesized product was target product. The product had good chemical reactivity and compatibility with epoxy resin. The thermo performance of modified epoxy resin system was improved in the proper ratio and curing mechanism, meanwhile the mechanical perfor-mance had been improved largely.