The S-shaped cylindrical series post-treatment includes DOC components, DPF components, mixers, SCR components and thermal insulation components. It features a compact structure, high conversion efficiency, low back pressure, convenient maintenance, low operating cost, and can effectively remove CH compounds, particulate matter and nitrogen oxides from engine exhaust. The mixer component can provide good uniformity of ammonia distribution, ensuring that the conversion efficiency of each catalyst meets the requirements and effectively avoiding urea crystallization. The thermal insulation and heat preservation components ensure the working temperature requirements of the internal catalyst, prevent high-temperature damage to surrounding electrical components, and protect personnel safety. It is widely applicable to the post-treatment systems of non-road four-mobile machinery.