Abstract: A group of welding thermal simulation tests for 13MnNiMoR steel were conducted to study the effects of welding heat input and peak temperature on the microstructure and impact toughness of the simulated heat affected zone (HAZ). The results show that with the increase of heat input, the impact toughness and hardness of the coarse-grained heat affected zone (CGHAZ) decreased due to the increases both in the volume fraction of granular bainite and in the size of bainite laths; with the decrease of welding peak temperature, a significant change in the microstructure and toughness was observed. Particularly, when the peak temperature was in the α+β intercritical region (800 ℃), polygonal ferrite with a large amount of martensite/austenite (M/A) constituent formed in the intercritical heat affected zone (ICHAZ). This microstructure led to poor impact toughness and made the ICHAZ become the weakest area in the HAZ.