The mechanisms of N2O formation during nitrogen oxides(NOx) reduction over close-coupled selective catalytic reduction(cc-SCR) catalysts for diesel engine exhaust were investigated by experiments on small-sample catalysts test combined with numerical simulations. Research results indicates that N2O generation at both low(170—300℃) and high(＞300℃) temperatures over cc-SCR catalysts. At high temperatures，N2O forms as a byproduct through the oxidation of NH3. However，at low temperatures，the N2O formation pathway differs from the ammonium nitrate(NH4NO3) decomposition pathway observed in conventional SCR systems commonly used in China Ⅵ diesel vehicles. Considering the absence of upstream diesel oxidation catalyst(DOC) for NO oxidation and the inhibitory effect of water vapor on NH4NO3 formation in the cc-SCR system，a low-temperature N2O formation mechanism via the non-ammonium nitrate pathway was introduced and the single-site kinetic model was utilized during the simulation. The modified reaction mechanisms incorporate temperature-dependent N2O formation，enabling accurate prediction of N2O volume fraction at a space velocity of 40000h-1 across the temperature range of 170℃ to 450℃. Furthermore，the impact of NH3 coverage on N2O formation was also studied. Results demonstrate that N2O formation rates increase with higher NH3 coverage. Additionally，O2 enhances N2O production at elevated NH3 coverage by promoting NH3 dehydrogenation.