Abstract: The water wall tube of supercritical boiler in a power plant leaked during operation. The causes of tube leakage were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, mechanical property test, scanning electron microscope and energy spectrum analysis. The results show that there were solidification cracks in the butt weld of water wall tube. Under the action of internal pressure stress and welding residual stress, the solidification cracks propagated continuously, formed transverse cracks through the tube wall, and finally led to the water wall tube leakage.