Abstract: The effect of Nb on the microstructure and property of simulated coarse-grained heat affected zone（CGHAZ）of Mg-treated steel was analyzed by thermal simulation tester, scanning electron microscope, impact toughness test. The results show that under the welding heat input of 300 kJ/cm, with the Nb content increasing, the orginal austenite grain size of CGHAZ decreases, and the fraction of grain boundary ferrite and blocky M/A constituents increases; the impact toughness of the CGHAZ decreases, while the hardness increases.