Abstract: A water wall tube explosion accident occurred in a power plant boiler during operation. The causes of tube explosion were analyzed by means of macroscopic observation, mechanical property test, chemical composition analysis, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that there was an incomplete fusion defect at the root of the seal plate of the water wall, and stress concentration was easy to form at the defect. Under the combined action of fatigue stress caused by boiler start-stop or frequent load fluctuation and thermal stress caused by temperature difference between sealing plate and water wall tube, fatigue cracks were formed in water wall tube. The cracks gradually expanded to the inner wall of the tube, and finally penetrated the tube wall, resulting in leakage accident of water wall tube.