Crystal facets can affect the catalytic decomposition of ammonium perchlorate, but the underlying mechanisms have long remained unclear. Here, we use the nanorods, nanosheets and nanocubes of CoO catalysts exposing {110}, {111} and {100} facets as model systems to investigate facet effects on catalytic AP decomposition. The peak temperature of high temperature decomposition (HTD) process (T) of AP by nanorods, nanosheets and nanocubes CoO decrease from 437.0 °C to 289.4 °C, 299.9 °C and 326.3 °C, respectively, showing obvious facet effects. We design experiments about AP decomposition under different atmospheres to investigate its mechanism and verify that the accumulation of ammonia (NH) on AP surface can inhibit its decomposition and that the facet effects are related to the adsorption and oxidation of NH. The binding energies of NH on the {110}, {111} and {100} planes calculated via density functional theory (DFT) are -1.774 eV, -1.638 eV, and -1.354 eV, respectively, indicating that the {110} planes are more favorable for the adsorption of NH. Moreover, the {110} planes are readily to form CoNO structure, which benefits the further oxidation of the NH.