The noise of exhaust muffler is one of the key factors of the noise of the whole car. As a key member in reducing the exhaust noise, its performance directly influence the smoothness of the car. By establishment of simulation model of coupling of engine working process and muffler the plug-in loss and pressure loss are obtained. The result of simulation calculation is basically the same as that of actual test. The measures to optimize the structure of the muffler proposed based on the study have achieved relatively satisfactory effect.