With the growth of the automobile consumption, the air pollution caused by automobiles has become a hot issue all over the world. Three-way catalytic converters are widely applied in the exhaust system to solve these problems, However, the pressure loss caused by these equipments should be discussed. In this paper, a common exhaust system is chosen as the object of study, and the internal with DOC and POC three-way catalytic converter structure is optimized by CFD numerical simulation, it is obtained that: (1)When the inlet expansion angle is 25°, the pressure loss caused by the combined three-way catalyst is relatively small; (2)When the inlet expansion angle is 25°, the exhaust back pressure of the entire exhaust system is relatively small, which has little impact on the engine; (3)The pressure loss caused by the combination type three-way catalyst directly affects the exhaust back pressure of the entire exhaust system, and the trend of the two changes is the same. The research work provides a theoretical basis and important reference for the structural optimization design of the three-way catalytic converter of the exhaust system.