Abstract: Eddy current testing standards at home and abroad are usually provided for through-hole defects in the signal phase for about 40 °. However, the actual detection of heat exchanger tube was found that the phase signal of leakage defects was as large as about 105 °, and as small as near 25 °. If the conventional phase-depth curve was used to evaluate the defect depth, the error would be very serious. The laws of signal phase of through-hole in relevant literature and the actual testing were analyzed, and it was found that through-hole defect signal phase was related to aperture size and wall thickness and such as volume factors. The more amount metal loss, the smaller the phase; and on the contrary, the less amount the metal loss, the larger the phase. So when testing the pipe defect with eddy current method, signal amplitude and phase should be combined to evaluate the defect size.