Abstract: The coal oxidation at normal temperature plays an important part in coal self-heating and the partition of "the three zones" of spontaneous combustion in goaf. In this paper, the real-time monitoring reactor was used to simulate the process of residual coal oxidation at ambient temperature in goaf. Based on the change of the oxygen concentration and the oxygen consumption rate, the mechanism of coal oxidation in goaf was analyzed. At the same time, the effect of hybrid particle sizes on the oxidation process was investigated. The experiments showed that the oxygen consumption rate of coal samples with hybrid particle sizes was not the linear superposition of each single particle size group, and the oxidation process of coal showed obvious phases with oxidation time increasing. According to the reaction mechanism in different phase, different method could be used for fire prevention.