Failure of heat exchanger at a wash of circulating water factory was analyzed by optical microscope, X-ray fluorescence spectrometer, X-ray diffraction (XRD), electrochemical test analysis method. The results show that chemical composition of heat exchanger didn’t meet the standard of 20# steel. Due to the surface of the vascular bundles scaling there is a serious corrosion. inside surface of tubes exist iron oxide layer which led to the under-deposition corrosion because different concentration of oxygen inside and outside deposition form corrosion battery. Therefore, under-deposit corrosion is the main the main cause of tubes perforation.