The phase of the reflected wave at the defect can characterize the change type of the defect. The frequency domain reflection (FDR) method can be used to locate cable defects based on the frequency from reflected waves, but cannot determine the sign of a reflected wave. In this article, a sign judgment method for the reflection wave at the defect is proposed to analyze the change type of the defect. First, the fast Fourier transform (FFT) interpolation algorithm of the Hanning self-convolution window is used to analyze the phase of the reflected wave. Then, the phase deviations are corrected to eliminate the effect of the limited starting frequency of the FDR method. Also, the frequency-dependent wave velocity is revised according to the defect location. The frequency value and the corrected phase value are used to obtain the propagation distance and the sign of the reflected wave, respectively. And the feasibility of the proposed method is analyzed based on the simulated cable models with single reflection wave and multireflection waves. Finally, the effectiveness and accuracy of the proposed method are verified through experiments on a 105 m 10 kV cross-linked polyethylene cable and a 10 kV distribution cable with a length of 1335.3 m.