Abstract: In this paper, cracking cause and mechanism of the water wall tube were investigated and analyzed by means of macroscopic morphology and metallographic observations, chemical composition detection and hardness testing. The results reveal that the cracking of the water wall tube is initiated from the intersection of the butt circumferential and fin longitudinal welds with high welding residual and bending restraint stresses composed of incomplete phase transformation structures. In addition, welding defects such as slag inclusions and pores in the matrix structure of circumferential weld due to unqualified process or improper operation are also found. Under the joint actions of alternating stress formed when the unit is started, stopped or heat load changed, welding residual stress and bending restraint force, crack is gradually formed and extended at the intersection of the butt circumferential weld of the water wall pipe and the longitudinal weld of fin. And finally, the cracking of the water wall pipe is caused with the accelerating expansion of the preexisting welding defects.