The quality of the insulation layer of high-voltage cables is crucial for the long-term reliability of power transmission and transformation systems. Non-uniformity in the thickness of the insulation layer can lead to localized tangential electric fields, which are prone to creating safety hazards. A reflective terahertz detection system, combined with a cylindrical coordinate scanning device, is employed to achieve full-angle scanning measurements of the insulation layer thickness of cross-linked polyethylene high-voltage cables. The non-roundness of the insulation layer thickness is quantified, and its distribution uniformity is assessed. The terahertz images intuitively display the interface wrinkle texture features of the insulation layer, which are highly consistent with the actual specimen morphology. This validates the effectiveness of terahertz imaging technology in the quality inspection of high-voltage cables, providing a new detection method and evaluation approach for the quality assessment of high-voltage cables.