The thermal stress and vibration characteristic of original structure of gasoline engine manifold catalytic converter are calculated with the fluid-structure thermal simulation. The heat transfer boundary conditions，such as the temperature of fluid boundaries and stress distribution，are calculated. Then，the thermal stress and vibration characteristic of original structure of manifold catalytic converter are calculated with Finite Element Analysis technology. According to the results，the thermal stress is excessive in entrance flange area. It is easy to produce the thermal stress concentration and is damaged easily in this area. In addition，the thermal deformation is large in the junction area between exhaust manifold and catalytic converter. Finally，the improved structure is presented according to the calculation results. The corresponding analysis and comparison show that the improved structure is better than the original structure in the highest temperature，the maximum thermal stresses and the vibration characteristic.