The combined influence of Mg and Ca treatment on the properties of heat-affected zone (HAZ) of low-carbon steel after high heat input welding was systematically studied. Experimental steels deoxidized with different elements were prepared, i.e., C–Mn steel with Al, Ti–Ca steel with Ti and Ca, Ti–Mg–Ca steel with Ti, Mg and Ca. Results showed that the inclusions in C–Mn steel were mainly Al2O3 and MnS with low density and large size. However, the average size was refined to only ~ 0.34 μm in Ti–Mg–Ca steel and the amount increased remarkably. Microstructure of simulated HAZ for 200 kJ/cm changed from ferrite side plates or upper bainite to acicular ferrite after treatment with Ti, Mg and Ca. Ca addition decreased the strain field around inclusions and enhanced the ability of acicular ferrite nucleation. In situ observation of Ti–Mg–Ca steel showed that the movement of austenite grain boundaries was retarded and nucleation sites of acicular ferrite were greater than Ti–Ca steel because of Mg addition. Impact energy of HAZ at − 40 °C was increased from 7 to 232 J and showed excellent stability because of Ti–Mg–Ca treatment. High volume fraction of acicular ferrite acted as obstacles toward cleavage cracks.