To improve the thermal ageing resistance of crosslinked polyethylene (XLPE), different contents of anti-ager 2-mercaptothiazole (MB) were blended with PE, and then MB/XLPE sheet samples were prepared through electron irradiation process. The effect of MB content on the crosslinking degree, thermal elongation performance, insulation properties, oxidation induction period, and carbonyl index of XLPE were studied. The results show that with the increase of MB content, the gel content and thermal elongation performance of MB/XLPE composites reduce, while the volume resistivity maintains stable. The addition of MB significantly extends the oxidation induction period and suppresses the carbonyl index growth of XLPE during thermal ageing. When the weight part of MB is 1, the oxidation induction period of MB/XLPE significantly increases from 0.9 min (for pure XLPE) to 80 min; after thermal ageing for 168 h at 165℃, and the carbonyl index of MB/XLPE only increases from 0.04 to 0.06, indicating marked improvement in thermal ageing resistance. Based on the Arrhenius equation, it is concluded that this MB/XLPE exhibits a service life of 76.2 years at 90℃ which is 2.5 times longer than that of conventional XLPE.