To investigate the influence law of particle size on the impact sensitivity of explosives in micro-and nano-scale and mechanism of action, the polymer-bonded explosives(PBXs) samples with apparent density as 1.74 g·cm^-3 and 1.50 g·cm^-3 were pressed from octogen(HMX, 100-300 nm, 1-2 μm and 10-20 μm) based PBX molding powder. The effect of HMX particle size and apparent density on the compression mechanical properties, thermal conductive properties and tablet impact sensitivity of PBX samples were studied. The micromorphology of samples before and after impact with drop hammer was observed by SEM, and the histories of force acting on samples during drop hammer impact loading were measured by a self-designed test device. Results show that the compressive strength, compressive modulus, thermal conductivity and thermal diffusivity of PBX increase dramatically with decreasing particle size of HMX. In same apparent density, the impact sensitivity of PBX decreases significantly with decrease of HMX particle size; for PBX samples with HMX of same particle size, the impact sensitivity increases slightly with the increase of apparent density. Under impact action, HMX samples with larger particle size mainly happen the pore compression and particle crack and HMX samples with smaller particle size mainly happen the shearing slip of particle layer. Under same impact condition, the peak of force acting on sample increases with increasing apparent density or decreasing particle size. Considering that the decrease of impact sensitivity caused by explosive particle refinement is the result of the combined effect of the edecrease of pore size, uniformity of pore structure, and enhancement of thermal conductivity etc. factors.