In-situ combustion (ISC) technol. is a promising method to exploit heavy oil.In this study, 1D tube combustion experiments and the thermogravimetric anal. were combined to investigate the combustion characteristics of the ISC process at the temperature range of 100-600βC.First, the temperature intervals of the three combustion stages of the ISC process were determined by thermogravimetric anal. and examined by the 1D tube combustion experimentThen the activation energies of the three stages were estimated through the Flynn-Wall-Ozawa (FWO) method and the Kissinger-Akahira-Sunose (KAS) method.Results showed that after the ISC reactions, the gravity of the crude oil was upgraded, and the key to maintain the ISC process is keeping the crack reactions continue.The activation energy varies between 66 and 75 kJ/mol in low-temperature oxidation, 197-218 kJ/mol in fuel deposition, and 92-109 kJ/mol in high-temperature oxidationThe different characteristics of the three stages were traced to the occurrence of different reactions, and the high activation energy value of the FD stage was traced to the high proportion of the resins and asphaltenes fractions of the crude oil.Conclusions of this work can help to understand the ISC process better.