The layout of closed-couple catalyst converter in internal combustion engine exhaust system is one of important way to reduce vehicle emission. CFD method based on finite volume method is adopted to numerical simulate flow distribution in the entrance of closed-coupled catalytic converter and applying business software of FLUENT to clarity the flow uniformity of inlet to ensure catalytic converter work efficiently and meet regulations. The flow field uniformity of entrance were studied and analyzed. Research finds that the shape of manifold has effect on dispersion of velocity in entrance and dispersion of velocity has a relatively strong correlation with pulsation flow. The flow reverse in junction deflect the air flow that flow into the entrance of closed-couple catalyst converter. Calculation results indicate that the uneven rate coefficient is 0.266 and volatility is 0.515 in the entrance of closed-couple catalyst.