Abstract: Leakage failure of BFe10-1-1 cupronickel tube of heat exchanger in an ultra supercritical thermal power unit occurred after four years operation. By means of macroscopic observation, chemical composition analysis, metallographic examination, fracture analysis and inner wall sediment test, the failure causes of the cupronickel tube were analyzed. The results show that there were obvious deposits on the inner wall of the cupronickel tube, and oxygen concentration corrosion was formed under the deposits, which resulted in pitting corrosion on the inner wall of the tube. Cl^- in the pitting corrosion pit area further corroded the iron elements at the grain boundary, which makes the iron elements enriched at the grain boundary lost continuously and formed vacancy. The continuous formation and combination of vacancy caused to the initiation of intergranular cracks, which eventually led to intergranular corrosion cracking of the tube.