Abstract: In order to ensure the reliability of the eddy current inspection of nuclear fuel cladding tube, the influence of frequency on the amplitude and phase of the inspection signal is studied by using the defect signal in eddy current inspection. According to the principle of eddy current inspection, the relationship between the eddy current density in cladding tube and the standard penetration depth was obtained, and the phase lag curve of cladding tube was obtained through the test. The experimental results show that, the defect depth can be accurately evaluated by combining the phase information of the defect signal in eddy current inspection.