Abstract: In this paper, the main bearing housing bore and main bearing of a diesel engine were studied, and the abstract: The non-road National Ⅳ emission standard will be released and implemented in December 2022.Compared with the National Three, the National Ⅳ standard has stricter restrictions on NO_x and PM.In response to the National Ⅳ emission regulations, the main technical route in the industry is to add an oxidation device（DOC）,a filter device（DPF） and a reduction conversion device（SCR） in the exhaust system, respectively, to realize the reduction of HC,CO,PM and NO_x in the exhaust gas control.In this paper, an evaluation method for the design and evaluation of a post-treatment scheme is established.Taking reducing exhaust back pressure, reducing the risk of urea crystallization, and improving the mixing uniformity before the carrier as the optimization indicators, under the constrained space, it guides the structure of the pipeline and mixer of the post-treatment system.And the layout and other aspects are optimized to improve the performance of the post-processing system.