Abstract: The leakage of high temperature superheater tubes of a 600MW supercritical boiler were analyzed by means of macroscopic inspection, chemical composition analysis, metallographic structure inspection and mechanical property analysis.The results show that the microstructure, chemical composition, tensile properties and hardness of the superheater tube near the leak point are normal. During the operation of the boiler, due to the inconsistency of expansion, deformation and displacement of the tubes on the high-temperature heating surface, the connecting part between the sliding positioning block of the tube row and the tube are subjected to great stress. Meanwhile, the welding of sliding positioning block and pipe adopts single side welding, there is stress concentration at the junction of the unwelded side of the weld and the base metal. After long-term operation, the crack is initiated at the stress concentration part of the weld and extends to the base metal, which causes leakage.