Partial discharge (PD) detection can be used for fault location and insulation state detection and has been widely used for power cables. However, during the PD test, the traditional time-domain reflection method has the defects of signal attenuation and low accuracy. At the same time, it is difficult to obtain accurate defects due to the limited propagation distance of partial discharge signals and the limited detection position such as in long-length submarine cables. Therefore, the traditional PD detection method cannot meet the application requirements of long-length cables under the development of new generation of smart grid. This article applied a non-destructive testing method, frequency domain reflectometry broadband impedance spectroscopy (FDR), to accurately obtain the defect or intermediate junctions location of submarine cables. FDR detection was carried out on a four phases 110kV cross-linked polyethylene insulated submarine cable. Data of the submarine cable was processed by the FDR method to analyze the reflection coefficient spectrum defect location curve. Since the abnormal peak position represented the defect or the joint position, the defect history at B phase and the spare phase of the submarine cable could be accurately located at 4.5km and 4.0km respectively. Using this test as a guide, a historical failure problem was discovered that the two-phase temperature measuring optical fiber of the submarine cable had been damaged and fractured after being squeezed. The test results provide an effective basis for improving the status diagnosis capability and fault finding level of long-length submarine cables.