Abstract: Selective catalytic reduction (SCR) systems are typically used to reduce nitrogen oxides (NO_x) emitted from diesel engine exhaust emissions. To further improve its de-NO_x performance, firstly, a three-dimensional numerical simulation model was established. The non-uniform distribution of the catalyst was achieved by changing the porosity in the SCR converter, and the impact of the non-uniform distribution of the catalyst under different engine loads on the flow, mass transfer, and heat transfer performance of the SCR system was investigated. Secondly, the impact of the non-uniform distribution of the catalyst on the performance of the SCR system was examined and compared with that of the uniform distribution. Finally, based on the principle of field synergy, the influence of velocity and temperature fields on the heat transfer process in the SCR system was analyzed. The results indicate that the non-uniform distribution of catalysts can improve the performance of SCR systems. At 25% engine load, compared to a uniform distribution, the pressure drop of Case P-R5 is reduced by 165 Pa, the conversion efficiency of NO_x is increased by 0.8%, and the escape of ammonia is reduced by 7 mg/L. In addition, the temperature gradient of Case P-R5 is higher than the maximum peak of the temperature gradient of the catalyst with uniform porosity, indicating that there is less energy transferred to the outside. The non-uniform distribution of the catalyst structure can maintain the temperature of the catalytic region, improve the catalyst activity, and facilitate de-NO_x reactions.