The spray and decomposition process of urea solution and chemical reaction process on the surface catalyst of selective catalytic reduction(SCR) system were simulated by using computational fluid dynamics(CFD) method. Combined with SCR engine test, the influences of ammonia nitrogen ratio, temperature, space velocity and urea injector position on NO conversion efficiency were researched. The experimental results show that NO conversion efficiency first increases and then decreases when the ammonia nitrogen ratio is between 0.6 and 2 and reaches the peak when the ratio is 1.2. NO conversion efficiency increases among the range of 300450 ℃ and becomes stable when the temperature reaches 450 ℃ till to 500 ℃. At the space velocity range of 13 00021 000 h^-1, NO conversion efficiency decreases with the increase of velocity. In addition, the increase of distance between injector and catalyst will lead to the increase of NO conversion efficiency, but has little influence on it beyond 450 mm.