Abstract: During the eddy current detection of the heat transfer pipe of a steam generator in a domestic nuclear power plant, some suspicious signals were found, and the type of defect could not be determined. Therefore, a simulated sample was designed and fabricated, and axial differential probe was used to carry out eddy current detection and comparison tests on the simulated sample. At the same time, rotary probes and array probes were used for review, which simulated conventional ultrasonic is impossible. The solution when effectively distinguishing the signal. The test results show that the defect is not just a dent signal, but a compound signal of dent and defect. For similar signals, the interference variables can be eliminated by mixing frequency to achieve the purpose of differentiation and quantification.