Abstract: In order to study the formation pathway of N₂O in NH₃-mediated selective catalytic reduction (NH₃-SCR) of NO_x process and the variation of N₂O selectivity affected by pressure, in situ synchrotron radiation photoionization mass spectrometry (SR-PIMS) and flow tube reactor were used to detect the gaseous species in the NH₃-SCR reaction over 3Mn10Fe/Ni catalyst. The variations of N₂O selectivity, NO_x conversion and NH₃ conversion were analyzed under different conditions. The results show that the formation of N₂O is mainly from non-selective catalytic reaction (NSCR) and adsorbed NH₃ oxidation (NSNO). Among them, NSCR plays a dominant role in N₂O formation in the temperature range of 100−250 ℃. The contributions of the two formation pathways in the temperature range of 250−400 ℃ are equivalent, and NSNO is the main source in the temperature range of 400−500 ℃. In addition, low pressure reduces the denitration activity of the catalyst at low temperature, but promotes the formation of N₂O via NSNO reaction at high temperature.