Abstract: Cast austenitic stainless steel (CASS) is widely used in the primary pipeline of pressurized water reactor nuclear power plants. Cast stainless steel will undergo thermal aging and embrittlement during long-term high-temperature service. CASS is actually composed of austenite and ferrite, and after long-term thermal aging, ferrite undergoes amplitude modulated decomposition, with the precipitation of G phase in the matrix. These are the main reasons for the embrittlement of CASS materials. During actual service, the main pipeline of the primary circuit not only bears the effects of long-term high-temperature thermal aging, but also bears the cyclic thermal and mechanical loads caused by startup and shutdown, reactor power fluctuations, and coolant flow, as well as the corrosion caused by the coolant of the primary circuit. These factors pose a risk of thermal aging and corrosion fatigue cracking in the main pipeline of the primary circuit. However, the impact of thermal aging on the performance of CASS mainly focuses on mechanical properties such as tension and impact, and there is relatively little research on the fatigue performance of CASS after thermal aging, especially in high-temperature and high-pressure water environments. Therefore, accelerated thermal aging tests were conducted at 400 ℃ of different periods on a typical nuclear power plant main pipeline material Z3CN20-09M, and the structure of the thermal aging samples was analyzed using transmission electron microscopy in this study. The fatigue life changes of Z3CN20-09M after thermal aging were studied in high-temperature air of 300 ℃ and simulated high-temperature and high-pressure water environment of the primary circuit to analyze the effect of thermal aging time on the fatigue behavior and life of Z3CN20-09M. The HRTEM results indicate that Z3CN20-09M undergoes lattice distortion on its (011) crystal plane after 5 000 hours of thermal aging. No amplitude modulated decomposition products, namely Fe rich α phase and Cr rich α' phase, were found in Z3CN20-09M after 5000 hours of thermal aging, and no G phase was found. The peak stress changes of Z3CN20-09M before and after thermal aging during cyclic loading are mainly divided into cyclic hardening stage and cyclic softening stage. After 1 000 hours of thermal aging, the fatigue life of Z3CN20-09M under high temperature air and simulated high-temperature and high-pressure water environment of the primary loop decreases slightly. As the thermal aging time extended to 5 000 hours, the fatigue life of Z3CN20-09M decreases. The ASME fatigue design curve still has sufficient safety margin for evaluating the fatigue life of Z3CN20-09M after thermal aging.