Abstract: Catalytic decomposition of NO by Cu-ZSM-5 has potential application. In order to reveal the catalytic decomposition mechanism of NO over Cu-ZSM-5, the adsorption of NO over short-range Cu⁺ pairs in Cu-ZSM-5 was simulated based on density functional theory. The reaction pathways of NO decomposition assisted by the by-products N₂O and NO₂ were also proposed. The results showed that the double nuclear copper-oxygen species was an important active centre. During the reaction, the highest activation energy (171.39 kJ/mol) was required for the decomposition of the by-product NO₂ on the binuclear copper-oxygen species. While that for the decomposition of N₂O was 86.92 kJ/mol, suggesting that the decomposition of NO₂ was more difficult. The desorption energy of N₂ and O₂ were 28.43 and 100.78 kJ/mol, respectively. The rate determining step was O₂ desorption. NO acted both as a reactant and a key reductant for the redox cycle of the active centre of Cu-ZSM-5 during the process.