In order to study the local defect type diagnosis method of low-voltage cable, this paper carries out frequency domain reflectometry (FDR) tests on a low-voltage cable sample with local defects, and studies the distortion characteristics of the FDR waveform of the sample at different time. Two knife mark defects of D1 and D2 (located at 8.7m and 11.8m respectively) are made at different positions of phase C of a low-voltage cable sample (14.1m), and the D2 defect of the sample is subjected to an accelerated aging in NaCl solution for 6 days, 9days, 12 days and 15 days, and then FDR tests are performed on the sample at different aging time. The test results show that the distortion position of the D1 defect on the FDR waveform does not change with the aging time. While the distortion position of the D2 defect moves to the left (of the test side) with aging. After 15 days of aging, the D2 defect distortion position moves 0.9 m to the left side. The analysis shows that the D1 defect is only a knife mark defect, the characteristic impedance of it does not change with aging, so its distortion position on the FDR waveform does not change with time. While the D2 defect is aged in solution, water diffuses to the insulation on both sides of the defect with aging, which leads to the left shift of the impedance mismatch point of the D2 defect and its distortion position.The study shows that the local defect location and type of the low voltage cable can be judged according to the FDR waveform distortion characteristics with aging.