Abstract: A self-designed Mn-Ni-Mo-B alloy welding wire was used to weld low-carbon bainitic steel by double-sided welding. The microstructure and mechanical properties of the welded joints were studied by optical microscopy, scanning electron microscopy, tensile testing, impact testing and hardness testing. Results show that the microstructure of weld and HAZ were mainly acicular ferrite and granular bainite. The HAZ with coarse grains showed a little softening. Under the condition of relatively small welding line energy, the welded joints had a good matching of strength and toughness. The tensile strength was 803.63 MPa while impact toughness of the weld and HAZ was 193 J and 232 J.