The relationship between the types of active sites and the selective catalytic reduction (SCR) activity of NO with NH3 over CuMn2O4 spinel was established through density functional theory (DFT) calculations. A skeletal reaction scheme including the possible elementary steps was proposed to understand N-2, NO2 and N2O formation during NH3-SCR of NO over CuMn2O4 catalyst. DFT calculation results show that chemisorption mechanism is responsible for the adsorption of reactants, possible intermediates and products over CuMn2O4(100) surface. 2-fold coordinated surface Cu atom plays a crucial role in NH3-SCR of NO, because it is the active site for NH3 and NO adsorption. NH2 produced from NH3 dehydrogenation is identified as a key reactive intermediate of SCR reaction. NH2 easily reacts with the adsorbed NO to form N-2 and H2O via NH2*+ NO* -> N-2*+ H2O* which is activated by 6.87 kJ/mol. The activation energy barrier of N2O formation over CuMn2O4 catalyst is much higher than that of N-2 formation, which indicates that CuMn2O4 catalyst shows a good N-2 selectivity for NO reduction. The optimal reaction pathway for NH3-SCR of NO over CuMn2O4(100) surface is a two-step process controlled by NH2*+* -> NH2*+ H* and NH2*+ NO* -> N-2*+ H2O*. The rate-determining step of N-2 formation during NO reduction is the first dehydrogenation reaction of NH3.