Abstract: In a supercritical boiler, 12Cr1MoV water wall cracked and leaked after repair welding. In order to avoid recurring of similar issues, the causes of crack in water wall after repair welding were analyzed by means of macroscopic morphology observation, chemical composition detection, metallographic analysis, hardness measurement and EDS analysis.The results show that the crack are mainly distributed in the hardened structure of the repair welding position, mainly intergranular crack. The hardness of some areas of the repair welding exceeds the upper limit value of the hardness of the weld in standard. It is inferred that the cracking form is welding cold cracking. The main reason for the failure is that the 12Cr1MoV steel itself has larger harden quenching tendency, and the welding process is controlled improperly, so that the cold cracking tendency of the weld is further increased. The weld has a hardened structure with excessive hardness, the residual stress increases, and the cracking occurs. It is suggested that the field welding process should be strictly controlled, the welding process guidance documents that meet standards should be formulated before welding, and adopt the measures such as cleaning groove, preheating before welding, drying and heat preservation of welding materials, heat input control, slow cooling or hetat treatment after welding,etc. It can avoid the welding cold cracking of 12 Cr1MoV water wall.