Abstract: As a kind of the medium-carbon quenching and tempering steel(MCQTT), 30CrMoSi has been used widely in some industrial fields. However, just like some MCQTT, this steel faces such problems as cold cracking in the welded joints. In this research, the fracture and microstructure of the joints of 30CrMoSi steel welded by TIG method were studied by scanning electron microscopy(SEM) and optical microscopy(OM), respectively. More attention has been paid to the mechanism of the cold welding crack in 30CrMoSi steel. The results show that the hot welding cracks formed at high temperature are prone to inducing cold welding cracks. The potential method to control the cold welding crack in 30CrMoSi steel was discussed as well.