Diesel emission aftertreatment system is usually designed to meet stringent packaging constraints, rendering a difficult situation to achieve perfect flow distribution inside the catalytic unit. The non-uniform flow pattern leads to a mal-distribution of flow velocity, temperature, and gas species in catalyst unit. Some catalysts are exposed to harsh working environment, while the rest catalysts are underutilized. This lowers the efficiency of overall catalyst unit and thus requires an oversized system to meet emission requirements. The flow mal-distribution also accelerates the uneven catalyst degradation, lowering the system durability. Hence, a quantitative description of packaging impact on catalyst performance is critical to assess the system efficiency and durability.In the present work, a mapping method is developed to combine catalyst performance with computational fluid dynamics (CFD) simulation. This method is used to analyze the performance and robustness of a SCR aftertreatment system using a series of packaging designs. The simulation results are validated by the engine test. A sensitivity study is carried out over critical parameters, including uniformity index, catalyst size and ANR. The simulation results indicate that an increase in the uniformity of flow velocity and urea distribution leads to an increase in NOx conversion and a decrease in NH3 slip. The benefit of uniform flow pattern becomes more significant as the catalyst system degraded. This methodology could be applied to system with DOC and DPF as well.