Abstract: The dissimilar steel welded joint in the ultra-critical generator set pipeline is considered as a weak link, susceptible to mechanical property degradation or even failure under high temperature and pressure conditions. Eddy current testing (ECT) is an important method for evaluating metal material properties. In this study, ECT was employed to detect thermal aging in SUPER304/T92 dissimilar steel welded parts. Firstly, an eddy current sensor was developed and an experimental platform for eddy current scanning was constructed. Secondly, a frequency sweep experiment was conducted to analyze the influence of different excitation frequencies on the eddy current detection capability of the welded specimens. Eddy current detection signals were selected from three excitation frequencies and analyzed in conjunction with the metallographic microstructure of the specimens. Finally, the relationship between eddy current signals in three different areas (SUPER304, weld and T92) and thermal aging time was explored while correlating it with surface hardness measurements of the specimen. The research results provided an effective nondestructive testing method for evaluating thermal aging in dissimilar steel welds.