Abstract: The water wall tube of a supercritical 600 MW unit boiler in a power plant leaked during operation.The leakage causes of water wall tube were analyzed by macroscopic examination,inner wall scale measurement,chemical composition analysis,metallographic examination,scanning electron microscope and energy disperse spectroscopy analysis,mechanical property test and hardness test.The results show that the scaling layer on the inner wall of the water wall tube was thick and distributed unevenly,and the combustion of the boiler was not stable,which resulted in corrosive thermal fatigue cracks on the outer wall and thermal fatigue cracks on the inner wall of the water wall tube,and finally led to leakage failure.