In order to study the thermal decomposition of nano ammonium perchlorate (AP), the AP samples with nanometer structure were prepared by rapid freeze-drying method. The morphology, structure and thermal decomposition property of the samples were characterized and analyzed, the apparent activation energy of samples were also calculated by Kissinger formula, then the effect of nano structure on the thermal decomposition of AP was discussed. The results show that the nanostructured AP obtained by rapid freeze-drying has nanoscale network framework and good crystal stability. The peak thermal decomposition temperature of nanometer AP is 424.2℃, which is 4.4℃ lower than that of raw material AP. Compared with the raw material, the apparent activation energy of nanometer AP is 167.8kJ/mol, which is decreased by 114.8kJ/mol. The study indicate that the nanometer AP samples prepared by rapid freeze-drying have more surface active atoms and groups, the samples are easy to be activated, which promote the thermal decomposition of AP.