This paper is aimed at understanding the chemical properties and microstructures of coke during in-situ combustion of heavy oil. It was achieved by conducting a coke generation experiment in an indoor high-temperature and high-pressure reactor. To begin with, the organic elements of the generated granular coke samples were studied, in addition to the findings on its Fourier transform infrared spectroscopy (FTIR), rock pyrolysis, scanning electron microscopy (SEM), and CT three-dimensional reconstruction. The experimental results suggest that the coke generated herein by in-situ combustion of heavy oil oxygen content that increased by nearly three times. The coke saw enriched oxygen levels but more deficient carbon levels compared with crude oil, with significant structural break in long-chain fatty acids and intensified aromatization. Due to the introduction of oxygen, oxygenated functional groups in the coke existed mainly in such macromolecular forms as aldehyde, ketone, carboxylic acid, ester and alcohol. Degradable S2 was the leading organic composition in the coke, which continued to break down into hydrocarbons of smaller molecular weight at high temperatures, providing fuel for sustained in-situ combustion of heavy oil. Carbon residue in coke is a type of polar macromolecular compounds of heavy oil through pyrolysis and polycondensation under high temperature and high pressure that may fuel the in-situ combustion at high temperature in an oxygenated environment. Moreover, observations using SEM and CT also showed that pore spaces of the coke consisted of a series of organic holes with different sizes. A large number of inter-connecting holes form a network of pore spaces that are indeed conducive to sufficient contact with oxygen in the process of in-situ combustion. As shown from the findings, this research may facilitate a better understanding of the chemical properties and microstructures of coke generated via high temperature oxidation during in-situ combustion, and may serve as valuable supporting references for the further mechanism development of fire flooding and for adjustment of relevant development schemes.