There are many factors that affect the efficiency of NOx conversion in the diesel engine SCR (selective catalytic reduction) post-processing system. In this study, we researched the factors affecting the efficiency of NOx conversion by engine bench testing. The SCR system was applied to the Weichai WD615 engine exhaust. First, the system used DOC (Diesel Oxidation Catalyst)/DPF (Diesel Particulate Filter) of which DOC is a noble metal-containing filter, it lets the filter passes through converting nitrogen monoxide (NO) to nitrogen dioxide (NO2) for passive regenerations and to provide high hydrocarbon (HC) oxidation activity for active DPF regenerations, the DPF is a wall-flow filter used to trap the remaining soot that the DOC can not oxidize. The combined catalytic oxidation device improved the original SCR deNOx system, which we developed. The new catalytic oxidation apparatus has a simple structure and fundamentally solves the clogging problem of the SCR method and the defects led by the unstable gaseous ammonia generation. Secondly, we studied the influence of an increased urea injection on the conversion efficiency of NOx and the secondary pollution of NH3 at different speeds and torques of the engine. It was found that at the maximum removal rate, the downstream NOx concentration was only 1 ppm. The NOx conversion efficiency can reach 79.10% or more—even 99.90%. Finally, we used the KDS and the BOSCH urea injection pumps to analyze the effect of different pumps on the conversion efficiency of NOx. Hence, the matching diesel engine urea SCR post-processing system can reduce the emission concentration and pollution. There was little difference between the two pumps under most of the working conditions, although KDS was a little better than BOSCH under some individual conditions.