Abstract: Low-temperature oxidation (LTO) is the main reaction that affects fuel formation in the in-situ combustion process, which has important significance for the subsequent combustion propulsion and the successful extraction of crude oil. In this study, heavy oil was subjected to LTO reactions at different temperatures. Three types of reaction products with varying oxidation depths were characterized in terms of the number of oxygen atoms and the polarity of the molecule to reveal the low-temperature oxidation process of the heavy oil. Ketone compounds and acid polyoxides in the oil phase and deep oxidation products with a higher number of oxygen atoms in the coke were identified with increasing oxidation depth. The experimental results showed that the oxidation reaction of the heavy oil changed from kinetic-controlled to diffusion-controlled in the open oxidation system of the heavy oil as the oxidation depth increased. The oxidation reaction of the oil phase reached a maximum and stable value in oxygen content. The molecular compositions of the ketone compound and acid polyoxide did not change significantly with further increase in reaction temperature. The molecular compositions of the deep oxidation products with a higher number of oxygen atoms in the coke phase changed significantly. The coke precursor molecules with a lower oxygen content and condensation degree participated in the coke formation, and the oxidation reaction pathway and the complexity of the oxidation product component also increased.