With the continuous increases in transmission voltage levels, the thickness of high-voltage DC cables has grown progressively, posing significant challenges for cable manufacturing and installation. Consequently, reducing insulation thickness has become critically important. Achieving this goal requires optimizing the electrical properties of insulation materials. This study calculates insulation thickness for XLPE cables using conventional design methods based on multiple sets of DC and impulse design field strengths. By incrementally increasing the maximum operational design field strength, the insulation thickness is systematically reduced. The rationality of the optimized thickness is verified through comprehensive analysis of temperature field distribution, electric field distribution under various operating conditions, and electro-thermal aging lifetime. The results demonstrate that when the DC and impulse design field strengths of XLPE are increased by 5% and 10%, the insulation thickness can be reduced by 4.8% and 9.3% respectively, providing valuable guidance for developing insulation materials with enhanced electrical performance.