Abstract: A three-dimensional computational fluid dynamics simulation platform for diesel engine aftertreatment systems was established by using the software of CONVERGE. The coefficients of the models, including porous media and urea pyrolysis, were calibrated. Prediction and optimization of the performances of diesel engine aftertreatment systems in terms of pressure loss, velocity and NH₃ homogeneity at the selective catalyst reduction（SCR） inlet and anti-deposit ability can be achieved through the platform. Simulation and analysis of gas flow, and physical and chemical phenomena such as urea solution injection, decomposition, crystallization was conducted at multiple boundary conditions based on an in-line aftertreatment system of a 4 L diesel engine. Analysis was carried out on the influence of two types of SCR mixers and two types of urea nozzles on the exhaust gas flow. Results show that, compared with mixer 2, mixer 1 leads to slightly higher pressure loss, similar velocity uniformity at the inlet of the SCR catalyst and significantly higher NH₃ uniformity. Both mixers show the similar risks towards deposit. The use of air-assisted nozzle reduces the uniformity of NH₃ concentration and the deposition resistance of the mixer.