Abstract: Economizer tube of a power plant leaked during the second trial operation. The leakage cause of economizer tube were analyzed by macroscopic examination, chemical composition analysis, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that there were manufacturing defects in the economizer tube, widmanstatten structure and decarburization layer in the heat affected zone of the weld joint, which increased the brittleness of the weld joint, and the original crack defects expanded until the economizer tube leaked.