Abstract: In this study, the welding heat-affected zone (HAZ), specifically the coarse-grained region of SA508-Ⅲ steel, was simulated using thermal cycle simulation techniques. Charpy impact tests were conducted at various temperatures to construct the ductile-to-brittle transition curve. The results indicate a significant reduction in impact toughness in the HAZ compared to the base material, supporting the rationality of ASME nuclear codes in requiring impact toughness compensation for HAZ regions. Furthermore, the influence of different welding processes—represented by varying thermal cycle curves—and the effects of post-weld heat treatment (PWHT) were systematically investigated. It was found that both welding method and thermal treatment have notable effects on HAZ impact performance. In particular, the impact toughness of the simulated HAZ was significantly improved after heat treatment, demonstrating the effectiveness of PWHT in restoring toughness degraded by welding thermal cycles.