Abstract: Nowadays, cross-linked polyethylene (XLPE) cables have emerged as the mainstream choice for high-voltage cables due to their excellent electrical properties, convenient installation and operation. The defects and aging issues of XLPE have received widespread attention. To investigate the fundamental mechanism of XLPE cable insulation aging at the macroscopic level, this paper reviews the microscopic processes and contributing factors leading to insulation defects in XLPE. It examines the direct effects of cross-linking by-products, other impurities, and cross-linking processes on the insulation performance of XLPE during its production, as well as the adverse effects of long-term combined electrical-thermal aging, moisture ingress, and other factors on the insulation performance of XLPE high-voltage cables during operation. Finally, the prospects for improving XLPE insulation materials are explored.