Abstract: During the welding process of bridge steel structures, the temperature effect generated by the welding thermal cycle is a crucial factor leading to microstructural inhomogeneity in the heat-affected zone (HAZ) of welded joints, which may subsequently induce local embrittlement and stress concentration issues in steel structures. This study utilizes a Gleeble testing machine to physically simulate the welding thermal cycle, focusing on analyzing the influence of peak temperature and cooling rate on the microstructure and impact toughness of the HAZ of the base material, 18CrNiMo steel, used in bridges. The research results indicate that when the peak temperature of the welding thermal cycle exceeds 950°C, significant austenitic transformation occurs in 18CrNiMo steel during the heating stage, and an increase in peak temperature leads to significant grain coarsening in the HAZ material. During the welding cooling stage, when the t_8/5 time is less than 15s, the martensite content in 18CrNiMo steel increases, resulting in a decrease in impact toughness. Based on the above analysis, to effectively avoid local embrittlement in welded steel structures of 18CrNiMo, strict interlayer temperature control during welding and post-weld heat treatment measures are necessary.