The quality of the insulation layer in high-voltage cables plays a pivotal role in ensuring the long-term reliability of the power system. The local tangential electric field caused by variations in insulation layer thickness can eposes significant safety risks. In this article, we employ a reflective terahertz detection system, combined with a cylindrical coordinate scanning device, to achieve comprehensive, full-angle measurement of the insulation layer thickness in cross-linked polyethylene high-voltage cables. This method enables us to accurately calculate the out of roundness of the cable insulation layer and assess the uniformity of its thickness distribution. Furthermore, terahertz images visually depict the interface wrinkling texture characteristics of the insulation layer, with results that align perfectly with actual specimens. This validation underscores the efficacy of terahertz imaging technology in quality inspection of high-voltage cables and provides a novel detection approach for evaluating the quality of these cables.