Abstract: Computer fluid dynamics(CFD) is adopted to simulate the flow fields of a catalytic converter while researching the working process of an emission postprocessor. The velocity field and pressure field of exhaust gas are obtained. Methods of structural optimization of catalytic converter for improving gas flow and conversion efficiency are proposed. The pressure loss of catalytic converter is caused mainly at the locations of inlets and outlets of monoliths, and flow distribuation in the catalyst is almost flat due to honeycomb resistance. The results are verified by experiment.