Cross-linked polyethylene (XLPE) is extensively utilized as power cable insulation due to its superior dielectric properties. However, temperature rise during high-voltage cable operation critically degrades mechanical performance, necessitating enhanced thermal stability in XLPE. This study proposes increasing cross-linking density by blending low-density polyethylene (LDPE) with linear low-density polyethylene (LLDPE). The cross-linking density of XLPE was quantified through gel content analysis and thermal elongation tests, with subsequent correlation analysis to creep resistance. Results demonstrate that LLDPE incorporation improves cross-linking efficiency to some extent, peaking at 1 phr LLDPE content. Increased cross-linking density is associated with a decrease in creep deformation and improved creep resistance at high temperatures. A non-linear relationship exists between the concentration of LLDPE and the creep enhancement of performance, indicating that there is an optimal blending threshold that can maximize the thermal-mechanical stability of cable insulation.