In order to comprehend the ageing characteristics profoundly and evaluate the ageing degree of cross-linked polyethylene (XLPE) cable insulation,the XLPE cable insulation is subjected to accelerated ageing experiment at 135 oC. The variation of elongation at break is characterized by the tensile test. The carbonyl concentration and antioxidant contents in XLPE are measured by Fourier transform infrared spectroscopy. Results show that the ageing of XLPE cable insulation exhibits critical time characteristic,namely elongation at break of XLPE decreases slowly and then decreases dramatically with ageing time. Meanwhile the carbonyl concentrations also increase slowly at first and then increases rapidly. The critical time is 2016h when aged at 135 oC. While the content of antioxidant gradually decreases without inflection point. Based on the chain free radical theory,the thermal ageing kinetics model considering the reaction of antioxidant is established,and the variation of antioxidant content and carbonyl concentration during the thermal ageing of XLPE cable insulation are furtherly calculated by simulation. Simulation results are in good agreement with the experimental results. The results indicate that the antioxidant content could reveal the ageing degree of XLPE insulation.