Abstract: Aiming at the burst accident of water wall tube in a power plant, the causes of burst were analyzed by means of macroscopic observation, mechanical property test, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that the sudden change of the fillet weld structure of the sealing plate at the elbow of the water wall caused stress concentration, and cracks were formed in the heat affected zone at the root of the fillet joint. Under the combined action of structural temperature difference stress, unit start-stop, load fluctuation and fatigue stress, the crack propagated from the outer wall of the tube to the inner wall, and finally penetrated the tube wall, resulting in a burst leakage accident of the water wall tube.