Abstract: A leakage occurred in the water-cooled wall tubes of a ultra-supercritical 1 000 MW unit at a power plant. The causes of the leakage were analyzed using macroscopic observation, chemical composition analysis, mechanical property testing, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that the cracking at the welds of the front wall spiral water-cooled wall tubes and the transverse cracks near the welds were caused by thermal fatigue of the material. The transverse cracks in the left wall vertical water-cooled wall tubes formed due to the combined effects of thermal fatigue and high-temperature corrosion.