Abstract: The cracking behavior of 15 Cr Mo G butt joint for water wall of 350 MW supercritical concurrent boiler after 40 000 h operation was investigated by macroscopic examination,fracture analysis,hardness test and microstructure analysis. The results showed that inappropriate welding parameters and large restraint stress of butt joint in water wall results in numerous crystalline cracks generating in the butt weld. Under the long term operation of boiler,solidification cracks extended through the whole weld and led to the frequent cracking leakage because of the action of primary stress,secondary stress and restraint stress of welded joint.