Abstract: Because the gain bandwidth product of traditional spectrum testing instruments is limited, and the output voltage in the high-frequency range is too low, the signal attenuation is relatively serious when testing long distance cables, which leads to a certain distortion problem in the impedance spectrum in the high-frequency range. This paper proposes a cable fault diagnosis and location algorithm based on the digital reconstruction algorithm of pseudo trapezoidal wave excitation impedance spectrum. The influence of local fault impedance of power cable on cable input impedance spectrum under different fault types is analyzed. The attenuation characteristics of the excitation voltage signal at the head end and the power transmission characteristics of different voltage amplitudes are analyzed. A large capacity pseudo trapezoidal excitation system based on SiC high-speed inverter is designed to measure the impedance spectrum information of the highest frequency 7 MHz. The full coverage of 0.1 Hz~60 MHz wide frequency range is achieved by using the digital reconstruction method of impedance spectrum. Compared with the traditional low voltage sinusoidal excitation, the positioning result using the time-space conversion function has smaller interval oscillation and faster convergence speed. The defect location accuracy is improved by 80%, which verifies the effectiveness and accuracy of this method.