Abstract: Aiming at the problem that the damage of high-voltage multi-core cable in power system is difficult to detect accurately, the detection method by using ultrasonic guided wave was studied. The propagation characteristics of the guided wave in the cable were analyzed, and a complete set of nondestructive testing equipment for ultrasonic guided waves was studied as well in which the excitation and reception of the guided wave was realized by using the PZT probe. Fixtures for the fixed cable and the ultrasonic probe were designed and fabricated. All the above-mentioned ones were tested on a long cable. Wavelet transform was used to denoise and extract features on the reflection from the defect. The results showed that the signal to noise ratio of the signal was improved. The weak echo signal could be accurately identified. Defect detection for both ends of the transmitter within 20m was achieved.