Abstract: In order to reveal the effect of different inert gases in preventing coal spontaneous combustion and the law of flame retardation, the experiment of isothermal dynamic displacement of oxygen by inert gas in coal with different particle sizes is carried out, and the concentration of oxygen, displacement time, displacement volume, displacement rate and other parameters that characterize the displacement process of He, N2 and CO2 are analyzed. The experimental results are showed as follows.① The concentration change of oxygen displaced by inert gas can be divided into three stages. Firstly, the inert gas displaces the oxygen in the pipeline of the gas detector, and the volume fraction is about 21%. Secondly, the inert gas displaces free and partially adsorbed oxygen in the coal sample tank, and the oxygen concentration remains stable. Thirdly, the inert gas has completely replaced the free oxygen in the coal sample tank, and the oxygen concentration shows a negative exponential change. ② The time for different inert gases to displace oxygen in coal generally decreases as the particle size increases. The total time for CO2 to displace oxygen is the longest, followed by He, and N2 is the shortest. ③ For the same particle size, the amount of oxygen displacement shows an obvious inflection point. Before the inflection point, the amount of oxygen displacement basically increases linearly. After the inflection point, the amount of oxygen displacement increases slowly with time and finally remains stable. The accumulative amount of oxygen displacement under different inert gases decreases with the increase of particle size. The accumulative displacement of CO2 to oxygen is the largest, followed by He, and N2 is the smallest. ④ The oxygen displacement rate under the effects of different inert gases is characterized by first disorder and then stable decrease. Among them, the disorder stage of CO2 displacement is especially obvious. In the stable decrease stage of oxygen displacement rate, the rate of oxygen displacement basically decreases with the increase of particle size at the same time.