Cross-linked polyethylene (XLPE) is widely used as insulating material of high voltage power cables due to its excellent physicochemical and electrical properties. It is necessary to understand the thermal ageing of XLPE in order to guarantee the reliability of power cables. In this paper, the XLPE samples were subjected to thermal ageing at 155°C in air. The influence of thermal ageing on optical properties of XLPE and its structural origin were investigated by Ultraviolet-visible spectrophotometer (UV-Vis) and Fourier transform infrared spectroscopy (FTIR). It was observed that the color of XLPE samples changed from transparent white to yellow even red with ageing time. The UV-Vis absorption spectra showed that the absorption edge shifted from ultraviolet to visible region which was consistent with the color change. The optical band gap Eg calculated from UV-Vis curves was found to decrease after thermal ageing. The modification in optical properties of XLPE was attributed to the formation of conjugated clusters according to UV-Vis and FTIR results. FTIR spectra showed that the unsaturation and carbonyl groups increased with ageing time which contributes to the formation of conjugated cluster. The structural changes indicated the chain scission and oxidation of XLPE molecular chains by thermal ageing. These experimental results also provide a promising method for condition assessment of XLPE cable insulation based on optical properties of XLPE.