Abstract: The characteristics of urea injection and NO<sub<<i<x</i<</sub< conversion efficiency are closely related.A SCR test bench was built based on V<sub<2</sub<O<sub<5</sub<-WO<sub<3</sub</TiO<sub<2</sub< catalyst,and NO,NH<sub<3</sub< and O<sub<2</sub< feed streams were dosed by mass flow controllers.The effect of normalized stoichiometric ratio (NSR) and catalyst temperature on ammonia storage,ammonia slip and NO<sub<<i<x</i<</sub< conversion efficiency was studied.The results show that the influence of NSR and temperature on ammonia storage,ammonia slip and NO<sub<<i<x</i<</sub< conversion efficiency is obvious.The time of ammonia storage will be shorten with the increase of NSR,then the improving speed of NO<sub<<i<x</i<</sub< conversion efficiency is high.The NSR fix 2.2,the NO<sub<<i<x</i<</sub< reduction efficiency increase from 0 to 4.3% within 15 s,and the NO<sub<<i<x</i<</sub< reduction efficiency increase from 0 to 8.7% within 15 s when NSR set 3.0.The time of ammonia slip and ammonia storage are closely related,the time of ammonia slip will be shorten with the increase of NSR.The method of NSR from larger to the smaller can increase ammonia storage rate and optimize NO<sub<<i<x</i<</sub< conversion efficiency.Using this injection method to carry out the European transient cycle(ETC) test,the brake specific emission of NO<sub<<i<x</i<</sub< reduce from 8.26 g/(kW·h) to 1.91 g/(kW·h),and the average and peak value of ammonia are 5×10<sup<-6</sup< and 18×10<sup<-6</sup<,the NO<sub<<i<x</i<</sub< can reach the National V emission regulations.