A method for diagnosing weak defects in cables based on the difference enhancement of the reflection coefficient spectra is proposed because it is difficult to identify weak defects using the current cable inspection techniques due to the slight difference between their reflection coefficient spectra and intact cables. First, the internal link between the reflection coefficient spectrum and the defect degree is explained, and the mathematical model of the cable line′s reflection coefficient spectrum is constructed. Second, an analysis is conducted to compare the reflection coefficient spectrums of the complete cable and the weak defect. A weak defect location function based on differential spectrum enhancement is devised with the goal of identifying the distinguishable difference between the two, and a reflection coefficient spectrum recovery approach is suggested. Using the current detection data, this method can recreate the theoretical reflection coefficient spectrum of the cable in good condition. Defect characteristics are successfully improved, and the sensitivity and accuracy of defect identification are raised, by processing the difference between the restored intact reflection coefficient spectrum and the reflection coefficient spectrum that contains defects. Ultimately, simulation and field tests were used to confirm the method′s efficacy. The findings demonstrated that the technique could precisely detect weak cable flaws and increase diagnostic accuracy, with a localization error of less than 4%.