Abstract: Leakage occurred in the weld on the original positioning slider of steel pipe of boiler final superheater of subcritical unit in a power plant. The leakage causes of steel pipe were analyzed by macroscopic inspection, mechanical property test, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that phase transformation in the local pipe wall was caused by improper welding process during welding positioning slider of steel pipe. The creep resistance of steel pipe in heat-affected zone decreased after long-term operation, which was the main cause for leakage of steel pipe in operation.