Abstract: The domestic cast austenitic stainless steel (CASS) used for PWR main coolant lines faces thermal aging phenomenon during long-term service. The accelerated thermal aging experiment at elevated temperature of 400 ℃ for CASS was carried out, the decrease regularity of absorbed energy for CASS at room temperature was obtained using Charpy impact test, and the variation trend of CASS’s fracture surfaces of impact specimens at different aging time was investigated using scanning electron microscope. The experiment results show that after 15 000 hours of accelerated thermal aging, the domestic CASS gradually reaches a thermal aging saturation, and its absorbed energy reduces greatly, but still meets the assessment indicators for unaged state required by design specifications.