Abstract: Butt MAG welding (80%Ar+20%CO₂) of 440 MPa grade HSLA steel was carried out with two kinds of self-made solid core welding wires. By comparing and analyzing the microstructure, inclusion size and composition of the two kinds of welds, the reasons for the significant difference in low temperature toughness of the two kinds of welds were explained. The results show that the welds of J1 welding wire are composed of acicular ferrite、ferrite side plate、proeutectoid ferrite and M-A constituent, while the welds of J2 welding wire are composed of acicular ferrite and M-A constituent. The particle size distribution, size, quantity and composition of inclusions in the weld of the two kinds of welding wires are small, which is not the main reason for the significant difference in low-temperature toughness between the two kinds of welding wires. Compared with the weld of J1 welding wire, the content of acicular ferrite in the weld of J2 welding wire increases, the content of M-A constituent decreases, and the proportion of strip and block M-A constituent decreases, which is the main reason why its low-temperature toughness is significantly better than that of J1 welding wire.