Oil sands bitumen (OSB) is the key component of extracted oil sands, thus further investigation of the mechanism of OSB pyrolysis reaction would be helpful for the development and application of oil sands pyrolysis process. First, chemical structure parameters and thermogravimetric (TG) behavior of OSB were experimentally investigated by 13C NMR spectroscopy and TG analysis, respectively, to initially evaluate the correlation between chemical structure parameters and pyrolytic behavior. Further, the ATR–FTIR spectroscopy technology was used to experimentally characterize and calculate the structural parameters of OSB at different pyrolysis final temperatures, and the main thermal evolution rules of different functional groups during the pyrolysis reaction were obtained. Based on this result and by using the model fitting method, the pyrolysis of OSB was found to be a parallel reaction. Moreover, the kinetic calculation results obtained by Straink method and distributed activated energy model method also supported this result. The correlation between chemical structure parameters and activation energy was analyzed, and it was found that the degree of aromatization Y-factor could be used to characterize the pyrolysis reaction activity. Finally, this study proposed a simplified mechanistic model of chemical structure evolution during OSB pyrolysis.