The decomposition course of Ammonium Perchlorate (AP) particles under electron energy is observed using a G2 pro Desktop Scanning Electron Microscope. The state that the AP particle is starting to decompose exhibiting cracks on its surface is observed and recorded using the S-4800II Fielding Emission Scanning Electron Microscope (FESEM), the Image Pro Plus (IPP) System is employed to obtain the projected area of that decomposed AP particle in the FESEM image, and the critical decomposition energy of the AP particle is calculated. Results have shown that the critical decomposition energy is decreased with the reduction of AP particle size. Especially when the AP particle size is approximately under 20 mm, the critical decomposition energy is sharply decreased, and the critical decomposition energy is very small if under 3 mm. This result is very helpful to explain why the burning rate of propellants and the brisance of Fuel Air Explosives (FAE) are enhanced and the sensitivities of those AP-based energetic materials are increased with the reduction of AP particle size. Furthermore, this method can be used to calculate in an intuitionistic way the critical decomposition energies of other crystalline materials.