Abstract: An ultra-compact post-processing system, a supercharger assembly and an engine are provided. The ultra-compact post-processing system includes a mixer provided with an inlet joint to be connected to the supercharger body, a urea nozzle connected to the inlet joint, a shell provided outside a part of the mixer and provided with a gas outlet, and a catalytic disc provided in the shell between an outer periphery of the mixer and an inner wall of the shell and dividing an inner cavity of the shell into a mixing cavity and a gas-return cavity. The gas outlet of the shell is located at the gas-return cavity, and an outlet end of the mixer is provided with a mixing section that is located in the mixing cavity.