Abstract: During routine field inspection after overhaul of a certain type of aircraft, it was found that the eddy current flaw detection signal of the outer casing attaching lugs was abnormal, and there was no problem of relevant display in coloring flaw detection.Aiming at the suspected crack signal, the nondestructive testing methods such as fluorescence flaw detection and real-time digital radiography are further analyzed and discussed, and the stress-relief experiment and microscopic observation of heat treatment are carried out, and the final results show that the signal of suspected crack originates from the micro-cracks on or near the surface of the specimen, which is independent of the volume defects such as internal stress, near-surface inclusions and pore.Eddy current testing has high sensitivity and reliability for detecting micro-cracks on or near the surface of specimens.