Early pressurized water reactor nuclear power plants widely used cast austenitic stainless steel (CASS) for their main pipelines, mainly using shielded metal arc welding (SMAW) and semi-automatic welding methods with large grooves. The performance of the heat affected zone of the weld deteriorates more severely with service time, and significant thermal aging embrittlement occurs at high temperatures. This article conducts research on the thermal aging damage of manual welding seam materials in main pipelines, and completes tensile and Charpy impact tests of welding seam materials subjected to thermal aging at 375, 400, and 425 ℃ for 0, 1000, 3000, 6000, and 10,000 h. Predicting the aging time corresponding to the same degree of aging at different aging temperatures using the aging parameter method, as well as the aging time required when the Charpy impact energy of pipeline materials decreases to a certain value. When the base metal is at 375 and 400 ℃, the average value of activation energy Q is 159.8 kJ mol−1. when it is at 375 and 42 ℃, the average value of Q is 95.6 kJ mol−1. The average value of Q is calculated to be 106.4 kJ mol−1 when the heat affected zone is at 375℃ and 400℃; and the average value of Q is 90.3 kJ mol−1 when it is at 375 and 425 ℃. This study provides ideas and methods for evaluating the service life of primary pipeline welds.