Abstract: The leakage of water wall tubes occurred during the operation of a boiler. It was found that there were transversal cracks on the outer wall of dozens of tubes. The formation cause of the transversal cracks of the water wall tubes in the boiler were analyzed by means of macroscopic examination, chemical composition analysis, mechanical property test, metallographic examination, SEM analysis and so on. The results show that under the combined action of the thermal fatigue stress caused by the tube wall temperature fluctuation and the corrosion atmosphere, the boiler water wall tubes suffered corrosion fatigue on the fire side of the tube, which resulted in dense transversal cracks and eventually led to leakage of the water wall tubes.