The inerting effect of crystalline II type Ammonium Polyphosphate (APP-II) on explosion characteristics of micron-size Acrylates Copolymer (ACR) powders was experimentally studied. The inerting mechanism was analysed by combining thermogravimetry (TG) and differential scanning calorimetry (DSC) tests. The results indicated that the maximum explosion pressure (Pmax) and explosion index (Kst) was 10.4bar and 416bar m/s, respectively for ACR powders. The minimum explosion concentration (MEC) of ACR powders ranged from 20 to 30g/m3, and the experimental minimum ignition energy (MIE) of the ACR dust cloud was 10mJ. Therefore, ACR dust was determined to be severely combustible dust. There existed a minimum inerting concentration (MIC), and the explosion of ACR powders can be inerted completely by 80wt% APP-II. Furthermore, 30 and 40wt% APP-II had a significant inerting effect on the MIE of ACR dust. According to TG and DSC tests, thermal stability of ACR would be augmented by the introduction of APP-II. The addition of APP-II triggered lower maximum mass loss rate (MMLR), higher temperature corresponding to mass loss of 90% (T0.1), chars yield, and endothermic peaks. Consequently, the ACR dust explosion was inerted by the chemical interaction of ACR/APP-II mixtures and endothermic decomposition of APP-II.