Abstract: In this paper the nature,formation-mechanism and effective factors of the crack of ZGMn13 steel in HAZ were investigated by means of hot-ductility test and welding thermal simulation method.The results show that the crack of ZGMn13 steel in HAZ initiates by the liquation cracking.The main reason leading to the liquation crack in HAZ is that there are some kinds of low-melting phases formed by P,S and some other impurity elements in the steel and meanwhile there is a biger welding strees in HAZ.The non-metallic inclusions and carbides in the grain boundaries will propagate the liquation crack.The welding process tests show that the tendancy to liquation crack of ZGMn13 steel in HAZ will decrease when using the co₂ shielded arc welding with low hea input.