Abstract: The influence of preheating temperature,post-heating temperature and welding line energy on the tendency to cold cracking in welding GC-4 steel is investigated with implant test method and the highest hardness method.The microstruc-ture and fracture morpholgy in HAZ of the weld is observed with an electron microscope.It was proved that the influence of post-heating temperature is most significant.With a 300℃ preheating and a 350℃ post-heating, twinning Marten-site can restrained ,ocr can reach a high value of 1500 MPa,and cold cracking can be prevented.