In order to clarify the thermal reaction mechanism between iron/calcium (Fe₂O₃/CaO)-based oxygen carriers and coal char, the gas escape from the reaction process was resolved by TG-MS coupling, and the equivalent characteristic spectrum analysis (ECSA) method was used to determine the specific reactions occurring. Under Ar atmosphere, Fe₂O₃ starts to be reduced by graphite at 560°C, producing CO₂, and at 1040°C, CO is produced from the reaction between graphite and CO₂. When CaO is added to Fe₂O₃, CaO absorbs the CO₂ produced, affecting the path of the reaction. The reaction to produce CO₂ also occurs first in the two iron/calcium-based oxygen carrier conditions of sintered and pelletized ore, and CO production also occurs at high temperatures. In the case of air atmosphere combustion, only O₂ is consumed, and CO₂ is produced, no CO is formed. Graphite combustion consumes oxygen at a maximum value of 0.0393 mmol/min, and the addition of sintered ore reduces the maximum value of oxygen consumption to 0.0294 mmol/min, a decrease of about 25%, reflecting the oxygen buffering effect of oxygen carriers. Because the reaction of the coal char condition was concentrated at 300°C–700°C, this temperature interval, the addition of pelletized ore to coal char reduced the area of O₂ consumption from 3.36 to 3.01, indicating that the oxygen carrier provided additional oxygen in the reaction and reduced the oxygen consumption. The addition of iron/calcium oxygen carriers also reduces the reaction activation energy and makes the thermal reaction easier.