Ultraviolet (UV) ageing is an important factor affecting the service performance of asphalt pavement. However, at present, the group transformation of asphalt under ultraviolet radiation is still need to be studied. The ageing mechanism of asphalt could be better revealed through studying the functional groups of asphalt. In this study, asphalt were aged by three kinds of ultraviolet light. The changes of characteristic functional groups of asphalt before and after ultraviolet ageing were analyzed by infrared spectroscopy. Finally, the entropy weight method was used to calculate the count of inter conversion. The results showed that the carbonyl index of asphalt increased after ultraviolet ageing, while the methylene index, aromatic index, aliphatic index, and sulfoxide index decreased. After the asphalt is aged by ultraviolet, the C-C, C-H and C=C bonds in the asphalt material will be converted to C=O, and S=O will be converted to SO2. The results could be used to simulate asphalt molecules in order to better understand the performance and properties of asphalt.