Abstract: ACFM, which means Alternating current field measurement, is a NDT technique which detects cracks and determines the size of the cracks by measuring the change in the magnetic field. In recent years, ACFM equipment was introduced to the domestic marine engineering industries for underwater structure inspection. The paper described test cases where ACFM inspection by U31-type testing equipment was implemented for underwater ferromagnetic structures. The detected spectrums obtained were analyzed; the spectrum features of the different related display and non-display were summarized. At the same time, a defect determination process was prepared for the actual test to help identify defects.