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, it researched the factors affecting the efficiency of NOx conversion by engine bench test. The SCR system was applied to the Weichai WD615 engine exhaust. First of all, it 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. Combined catalytic oxidation device is to improve the original SCR deNOx system which was developed by ourselves. The new catalytic oxidation apparatus has a simple structure, and fundamentally solves the clogging problem of the selective catalytic reduction method and the defects led by the unstable gaseous ammonia generation. Secondly, it also studied the influence of the increase of urea injection on the conversion efficiency of NOx and the secondary pollution of NH3 under the different speeds and torques of the engine. It was founded that when the removal rate reaches the highest, the downstream NOx concentration was only 1 ppm. The NOx conversion efficiency can be achieved 79.10% or more, the maximum even reaches 99.90%. Finally, it used the KDS and BOSCH urea injection pump, to analyze the effect of different urea pumps on the conversion efficiency of NOx. Hence the matching diesel engine urea SCR post-processing system can reduce the emission concentration and reduce pollution further. It showed that there was little difference between the two pumps under most of the working conditions, and KDS was a little better than BOSCH under some individual conditions.