Abstract: Abstract: Selective Catalytic Reduction (SCR) is applied to a diesel engine for the promising solution of NOX emission. In this study, a diesel engine test bench with an SCR system was developed to clarify the large difference in the NOX emission of diesel engine SCR system under various exhaust conditions. A performance test of the SCR system was also used to determine the single and multi-factor interaction. An SCR model was established using GT-POWER software. A systematic analysis was made to explore the influence of exhaust temperature, exhaust mass flow, and ammonia nitrogen ratio on the SCR performance in the heavy-duty engine. Box-Behnken design and response surface method (RSM) was used to simulate the diesel engine SCR system. An RSM optimization was carried out with the NOX conversion efficiency and NH3 slip rate as optimization objectives, where the exhaust temperature, exhaust mass flow, and ammonia nitrogen ratio were variable factors. The results showed that the NOX conversion efficiency increased in the range of 150-250 °C, while a high level was then observed in the range of 250-450 °C, finally to decline after 450 °C. There was an opposite trend for the influence of exhaust temperature on NH3 slip rate. Specifically, the NH3 slip rate remained at a low level, all within 5% above 250 °C. The NOX conversion efficiency decreased, but the NH3 slip rate increased, with the increase of exhaust mass flow, especially when the exhaust mass flow was above 200 kg / h. When the exhaust mass flow increased by 50 kg / h, the NOX conversion efficiency decreased by 3%, and the NH3 slip rate increased by 4%. The high ammonia nitrogen ratio contributed to improving the NOX conversion efficiency and NH3 slip rate. Particularly, the NOX conversion efficiency maintained a high level, when the ammonia nitrogen ratio was above 0.9. Nevertheless, the NH3 slip rate maintained a low level, when the ammonia nitrogen ratio was below 0.9. It inferred that the appropriate ammonia nitrogen ratio was expected to optimize the SCR performance. In the response surface optimization, a high exhaust temperature and low exhaust mass flow with a suitable ammonia nitrogen ratio can contribute to the NOX conversion efficiency in the high level, while the NH3 slip rate in the low level. An optimal NOX conversion efficiency of SCR performance reached 96.4%, and the NH3 slip rate was only 0.5%, when the exhaust temperature was 350 °C, while the exhaust mass flow rate was 200 kg / h, and the ammonia nitrogen ratio was 1.0. Consequently, an optimal combination of NOX conversion efficiency and NH3 slip rate can be achieved under the optimization of exhaust temperature, exhaust flow and ammonia nitrogen ratio. This finding can provide effective guidance for urea control in an SCR system under different operating conditions of a diesel engine.