Refined coal tar pitch (QI＜0.2%) was used as the raw materials to produce coal-based needle coke. The quality of coal-based needle coke was decided by the thermal conversion properties of refined coal tar pitch. In this study, FTIR spectrum combined with curve-fitted method were used to quantitative analysis the structure changes of refined coal tar pitch in different thermal conversion temperature. The aromaticity index (Iar), branched index (CH3/CH2), contents of each basic function-groups (CO, CC, and C—O), and the species of aromatic substitution have been studied. The results showed that: The Iar and CH3/CH2 index of refined coal tar pitch increased with the increase of thermal conversion temperature. It meant that the rupture of the Branched chain may cause the production of active site, and the active site was one of the reasons to induce the aromatic rings increase. With the increase of thermal conversion temperature, the contents of CO decreased from 26.25% to 15.62%, the contents of CC improved from 43.39% to 51.28%, and the contents of C—O remained essentially unchanged. It means that, CO groups were another important reason to induce the occurrence of the condensation reaction of large molecule aromatic ring. The contents of 1H and 3H were decreased, but the content of 4H was increased, which indicated that, the aromatic substitution was also decreased, and the aromaticity improved. This phenomenon was match up to the Iar analysis.