The traditional time-domain reflection method is only applicable to more serious defects or faults because of its low content of high-frequency components, large signal attenuation in the transmission process in the cable, and the inability to compensate for the signal dispersion attenuation. It can not detect local weak defects well, and its positioning accuracy is low. Based on this, according to the characteristics of the reference signal, this paper proposes a positioning technology based on the waveform time-domain envelope. First, the reference signal and its reflected signal are enveloped by Hilbert transform in the time domain, then the envelope curve is logarithmized and normalized, and finally the eighth order Butterworth band-pass filter is used to filter and suppress noise. After that, 50 m and 500 m long power cables are actually tested, respectively. The results show that the time-domain envelope technology can effectively locate cable defects. At the same time, the type of cable defect can also be judged according to the change of time domain waveform.