Abstract: The sensitivity of welding cold crack of medium-Mn steel was investigated by a maximum hardness test and Y-groove welding cracking test through Ar-CO₂ gas shielded welding.The results show that when the welding heat input increased from 10 kJ·cm^-1 to 20 kJ·cm^-1 or the preheating temperature increased from 20 ℃ to 200 ℃, the microhardness of the welding heat affected zone of medium-Mn steel decreased slightly, and the maximum hardness was higher than 430 HV, indicating the steel had a severe welding cold cracking tendency.The surface crack and root crack of test weld in Y-groove welding cracking test disappeared and the section crack rate decreased to 9.09%, when the welding heat input was 15 kJ·cm^-1 and the preheating temperature increased from 100 ℃ to 200 ℃.In order to prevent welding cold crack formation, the preheating at 150-200 ℃ before welding medium-Mn steel and post-weld heat treatment should be carried out.The large martensitic lath in coarse-grained heat affected zone had little difference in crystallographic orientation and had a low density of large angle grain boundaries, which had weak resistance to cleavage crack propagation; thus, the welding cold crack extended along the coarse-grained heat affected zone adjacent the fusion line after initiation.