Abstract: CCT Diagram of 12Ni2CrMoV steel has been formulated through the investigation of the simulatad weld thermal cycles. The transformation principles and structure characteristics in the overheated zone have been observed. The Y-groove tests have been adopted in studing the effect of preheat temperature on the susceptibility to cold cracking. The test results have shown the critical preheat temperatures are 60℃ and 100℃ when the contents of diffusible hydrogen in the weld metal are 1ml/100g and 2ml/100g respectively. The energy imput may not exceed 29.3~33.4×10³J/cm in order to ensure a good notch toughness for the overheated zone. In consideration of the consistency of the data in both the CCT Digram and the Y-groove tests for 12Ni2CrMoV steel, the authors put forward the optimum welding technology for the fabrication of large welded structures.