In this paper, the cracking and leakage cause of the water wall tube was studied by means of macro morphology analysis, chemical composition analysis, microstructure detection, mechanical properties test and energy dispersive X-ray spectroscopy (EDS) analysis. The results indicated that the cracking of water wall reducer is mainly related to high temperature fatigue and sulfur corrosion. During operation, under the combined action of thermal expansion and contraction caused by frequent boiler load changes and axial alternating stress caused by pipe wall vibration, thermal fatigue cracks are gradually formed on the outer wall of the steel pipe toward the fire side. Under the combined action of the alternating stress and S corrosion, the cracks are gradually spread from outer to inner wall, and finally penetrate the tube wall, causing leakage. Finally, the corresponding preventive measures of water wall leakage are put forward to improve the safety and economy of power plant boiler.