In order to understand the effects of deoxidizer such as titanium, zirconium and magnesium on the impact toughness of heat affected zone (HAZ) of steel, four low carbon steels deoxidized by titanium, zirconium and magnesium were obtained. After smelting, forging, rolling and welding simulation, the effects of Ti, Zr and Mg additives on the impact toughness of HAZ in low carbon steel were studied. The inclusion characteristics (size, morphology and chemistry) analysis, Charpy-type test and the fracture observation of the specimens after the Charpy-type test were carried out respectively. The following results were found. The density of inclusion in Ti-Zr-Mg deoxidized steel is maximal. With the increasing of Ti content in steel, the proportion of inclusions in diameter less than 0.5 μm decreases. The proportion reaches maximum 64.0% in Ti-Zr-Mg deoxidized steel. The addition of Ti-Zr-Mg can enhance the impact toughness of HAZ after welding simulation. The maximal impacting energy is 249J at 253K. The complex particles of MgO-ZrO2-TiOx-MnS are most benefit to enhance impact toughness. The improvement of HAZ is attributable to the role of particle pinning and the formation of intergranular ferrite.