Abstract: The study aims to analyze the effect of inert medium on coal combustion and effectively prevent spontaneous combustion of coal dust. Thermal analyzer and gas chromatograph were used to measure changes of quality and concentration of CO and CO₂. The inert mediums are H₂O, NH₄H₂PO₄ and CaCO₃. Tests of single inert medium, NH₄H₂PO₄ and CaCO₃ with different proportions and particle sizes on the inhibition of combustion were conducted. The results show that the inhibition effect of NH₄H₂PO₄ and CaCO₃ mixture is better than that of H₂O and CaCO₃ but weaker than NH₄H₂PO₄ alone. The effect of inert medium in the process of instantaneous explosion and slow oxidation is not identical and there is no synergistic suppression effect on coal dust combustion. With the decrease of the size of NH₄H₂PO₄ , the inhibition performance is enhanced.