Flame propagation tests and explosibility tests were conducted on three different particle sizes of dust samples with a vertical glass tube and a 20 L spherical explosibility test apparatus. The explosibility was examined using Pmax, dp/dtmax, and tb. Proximate analysis and SEM analysis were performed on the oil shale dust samples before and after explosion. The results indicate that, within a given limit of particle size, the smaller the particle size, the greater the flame propagation velocity and explosibility of oil shale dust; beyond this limit, oxygen content and particle agglomeration will both make a difference, eventually weakened the combustion reaction. The extent to which particle size affects explosibility also varies as a function of concentration, and the optimum concentration of dust was found to vary with mass median particle size. The process of the oil shale dust explosion is mainly the volatile matter combustion reaction. Particle size affects oil shale dust explosion primarily as a result of the different pyrolysis rate and amount of volatile matter.