The thermal-oxidative ageing characteristics of cross-linked polyethylene (XLPE) are the critical factor affecting the operational life of cable insulation, but its internal relationship with XLPE cross-linking reaction conditions are still lacking in research. In this paper, seven types of XLPE samples were prepared by precisely controlling the cross-linking reaction temperature and time. Combined with Fourier transform infrared spectroscopy, tensile test, and gel content measurement, the influence law and mechanism of cross-linking reaction conditions on the thermal-oxidative ageing characteristics of XLPE cable insulation were investigated. The results show that the cross-linking reaction conditions directly affect the residual antioxidant content and the cross-linking structure in XLPE, while the thermal-oxidative ageing resistance of XLPE is dominated by the residual antioxidant content, which is also synergistically influenced by the cross-linking structure.When the cross-linking reaction temperature is too low or the time is too short, the cross-linking structure of XLPE is incomplete; when the cross-linking reaction temperature is too high or the time is too long, the cross-linking structure of XLPE will not be significantly improved, while the residual antioxidant content in XLPE will decreases. Both the two cases will lead to a decrease in thermal-oxidative ageing resistance of XLPE. The XLPE prepared under the cross-linking reaction temperature of 170℃ and reaction time of 16 min exhibits excellent thermal-oxidative ageing resistance, which is due to the high residual antioxidant content of XLPE under the cross-linking reaction condition and the perfect cross-linking structure. The research result provides a theoretical support for optimizing thethermal-oxidative ageing resistance of XLPE cable insulation.