Abstract: A fin tube of middle pressure evaporator for heat recovery steam generator of gas-steam combined cycle unit leaked during operation. Macroscopic appearance and microstructure observation, scanning electron microscope and energy spectrum analysis, phase analysis and laboratory scale dissolving test were used to analyze the leakage reasons. The results show that the structures under the leakage point and nearby corrosion product layer were normal. Local areas of some fin tubes had been severely corroded during the original manufacturing, forming the blister-like corrosion products, which cannot be removed by chemical pickling and dissolution. During the unit operation, the position was further corroded by oxygen and thinned, which eventually led to the leakage. It was suggested to strengthen the acceptance management of the fin tube of middle pressure evaporator, focusing on the original defects such as blister-like corrosion and scratches to prevent such leakage accidents from recurring.